Also since gzread() will no longer return an error for an incomplete
gzip file, have gzclose() return an error if the last gzread() ended
in the middle of a gzip stream.

Z_BUF_ERROR was also being used for an unsuccessful gzungetc and for buffer
lengths that didn't fit in an int.  Those uses were changed to Z_DATA_ERROR
in order to assure that Z_BUF_ERROR occurs only when a premature end of
input occurs, indicating that gzclearerr() can be used.

Before, gzeof() would return true (accurately) when the last read request
went just up to the end of the uncompressed data.  In the analogous case,
feof() would return false, only returning true when a read request goes
past the end of the file.  This patch corrects gzeof() to behave in the
same way as feof(), as noted in the zlib.h documentation.

inflate() avoided that allocation normally, until it was modified to
update the window on a normal completion so that inflateResetKeep()
could work.  This patch restores that behavior, but only when
Z_FINISH is used successfully to complete an inflation of a stream in
a single call of inflate().  The comments in zlib.h have been updated
accordingly.

gzwrite.c had hard-coded parameters to deflateInit2() which could
contradict compile-time options for the use of less memory and fewer
code bits.  This patch suggested by Karsten Saunte fixes that.

This patch adds the deflateResetKeep() function to retain the sliding
window for the next deflate operation, and fixes an inflateResetKeep()
problem that came from inflate() not updating the window when the
stream completed.  This enables constructing and decompressing a series
of concatenated deflate streams where each can depend on the history of
uncompressed data that precedes it.

This generalizes deflateSetDictionary() and inflateSetDictionary() to
permit setting the dictionary in the middle of a stream for raw deflate
and inflate.  This in combination with the Keep functions enables a
scheme for updating files block by block with the transmission of
compressed data, where blocks are sent with deflateResetKeep() to
retain history for better compression, and deflateSetDictionary() is
used for blocks already present at the receiver to skip compression but
insert that data in the history, again for better compression.  The
corresponding inflate calls are done on the receiver side.

Add a cover target in Makefile and the test/infcover.c test program
to cover all of the code lines in the inf*.c source files.  The
coverage is run with memory allocation checking in order to expose
memory leaks.  The coverage testing is run using:

    ./configure --cover && make cover

During coverage testing it was discovered that these two lines could
never pull more bits, since the immediately preceding for loop assures
that all of the code's bits are already pulled.

Due to earlier changes in the error checking in inflate_table(), the
code to fill in a table for an incomplete code handled cases that can
never actually occur.  This simplifies that code to handle the only
possible case, which is a single empty table entry for a code with
a single symbol with a length of one bit.

Using "ON" was a dumb idea, since it is common to have macros with
names like ON and OFF.  In fact, defining the OF macro back in 1995
was a bad idea, but now we're stuck with it.  Attempts to rename OF
to something else breaks many applications.

Adds ARFLAGS variable for options, where AR is now just the command
name.  So now $(AR) $(ARFLAGS) is used to build the static library.

Patch provided by Franz Schrober.

A problem surfaced in a multi-threaded application where fileno() was
used to get a file descriptor from an fopen(), which was then fed to
gzdopen().  The problem occurred when the gzclose() followed by the
fclose() tried to close the same file descriptor twice.  If fclose()
were not done, there would be a memory leak.  The only way out is to
dup() the file descriptor so that gzclose() closes the duplicated
file descriptor, and fclose() closes the original file descriptor.

This permits compilers to check for the proper treatment of next_in and
msg in the z_stream structure.  This is an option instead of the default
in order to preserve backward compatibility.  Some applications make use
of the z_stream structure outside of zlib, and perform operations such
as free(strm->next_in), which would not be permitted when next_in is
const.  The #define ZLIB_CONST needs to precede the #include "zlib.h">,
in order to make next_in and msg const pointers in the z_stream type.

This adds the -fprofile-arcs and -ftest-coverage options when compiling
the source code for the static library.  Those same options must then be
used when linking the static library into an executable.  This updates
Makefile.in to remove and .gitignore to ignore the files generated when
testing coverage.

The Microsoft CAB file format compresses each block with completed
deflate streams that depend on the sliding window history of the
previous block in order to decode.  inflateResetKeep() does what
inflateReset() does, except the sliding window history from the
previous inflate operation is retained.

A common request has been the ability to compile zlib to require no
other libraries.  This --solo option provides that ability.  The price
is that the gz*, compress*, and uncompress functions are eliminated,
and that the user must provide memory allocation and free routines to
deflate and inflate when initializing.

This also moves some of the same from zconf.h to gzguts.h. A new
function, gzflags(), was created to pass the compilation flags
related to vsnprintf usage back to zlibCompileFlags() in zutil.c.
In the process, various compiler configuration files were updated
to include gzflags(), as well as the new gzgetc_() function added
when the gzgetc() macro was introduced in a previous patch.