Limit read() and write() requests to sizes that fit in an int.
This allows storing the return value in an int, and avoiding the
need to use or construct an ssize_t type. This is required for
Microsoft C, whose _read and _write functions take an unsigned
request and return an int.

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.

gzsetparams() was using Z_PARTIAL_FLUSH when it could use Z_BLOCK
instead. This commit uses Z_BLOCK, which avoids emitting an
unnecessary ten bits into the stream.

In some cases the return values did not match the documentation,
or the documentation did not document all of the return values.
gzprintf() now consistently returns negative values on error,
which matches the behavior of the stdio fprintf() function.

You would still need to run zlib2ansi on all of the *.c files.

The function is only available if stdarg.h is available.

If the deflateInit2() called for the first gzwrite() failed with a
Z_MEM_ERROR, then a subsequent gzclose() would try to free an
already freed pointer.  This fixes that.

A gzopen() to write (mode "w") followed immediately by a gzclose()
would output an empty zero-length file.  What it should do is write
an empty gzip file, with the gzip header, empty deflate content,
and gzip trailer totalling 20 bytes.  This fixes it to do that.

Avoid the use of an uninitialized value when the write buffers have
not been initialized.  A recent change to avoid the use of strm->
next_in in order to resolve some const conflicts added the use of
state->in in its place.  This patch avoids the use of state->in
when it is not initialized.  Nothing bad would actually happen,
since two variables set to the same unintialized value are
subtracted.  However valgrind was rightly complaining.  So this
fixes that.

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.

When successful, gzputc would return the second argument.  If the
second argument were -1, gzputc would return -1 instead of the
character written, which was 255.  However the -1 would not be
distinguishable from an error.  Now gzputc returns 255 in that
case.

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.

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 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.