This is a problem in the odd case that the second argument of
LSEEK is a larger type than off_t. Apparently MinGW defines off_t
to be 32 bits, but _lseeki64 has a 64-bit second argument.

Also undo a previous commit to permit MinGW to use _lseeki64.

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.

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.

An open() with O_APPEND followed by an lseek() to determine the
position will return zero for a non-empty file, even though the
next write will start at the end of the file.  This commit works
around that by doing an lseek() to the end when appending.

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

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.

The conversion to multi-byte will be locale-specific, but it's
better than nothing and is only to provide more information in the
error message returned by gz_error().  The conversion has no
effect on what's opened.

Also need to #include <stddef.h> for zlib.h, and need to workaround
the inability to use wide characters in constructed error messages
with zlib's interface.

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.

Before this fix, gzread() would lose data if a premature end of file
was encountered.  This prevented gzread() from being used on a file
that was being written concurrently.  Now gzread() returns all of the
data it has available before indicating a premature end of file.

This also changes the error returned on a premature end of file from
Z_DATA_ERROR to Z_BUF_ERROR.  This allows the user to determine if
the error is recoverable, which it is if Z_BUF_ERROR is returned.  If
a Z_DATA_ERROR is returned, then the error is not recoverable.

This patch replaces the functionality of a previous patch that fixed
reading through an empty gzip stream in a concatenation of gzip
streams.

To implement this fix, a noticeable rewrite of gzread.c was needed.
The patch has the added advantage of using inflate's gzip processing
instead of replicating the functionality in gzread.c.  This makes the
gz code a little simpler.