Normally these are set to size_t and ssize_t. But if they do not
exist, then they are set to the smallest integer type that can
contain a pointer. size_t is unsigned and ssize_t is signed.

This patch allows zlib to compile cleanly with the -Wcast-qual gcc
warning enabled, but only if ZLIB_CONST is defined, which adds
const to next_in and msg in z_stream and in the in_func prototype.
A --const option is added to ./configure which adds -DZLIB_CONST
to the compile flags, and adds -Wcast-qual to the compile flags
when ZLIBGCCWARN is set in the environment.

This avoids warnings in OpenBSD that apparently can't be turned
off whenever you link strcpy, strcat, or sprintf.  When snprintf
isn't available, the use of the "unsafe" string functions has
always in fact been safe, since the lengths are all checked before
those functions are called.

We do not use strlcpy or strlcat, since they are not (yet) found on
all systems.  snprintf on the other hand is part of the C standard
library and is very common.

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