to report the count of unused bytes at the end correctly.  Add the
ability to provide initial input to blast().

Also correct typo for 64-bit debug build.

Without this fix, it would be possible to construct inputs to puff
that would cause it to segfault.

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.

gzflags() was put in gzwrite.c in order to be compiled exactly the
same as gzprintf(), so that it was guaranteed to return the correct
information.  However that causes a static linkage to zlib to bring
in many routines that are often not used.  All that is required to
duplicate the compilation environment of gzprintf() is to include
gzguts.h.  So that is now done in zutil.c to assure that the correct
flags are returned.