This permits deflateParams to change the strategy and level right
after deflateInit, without having to wait until a header has been
written. The parameters can be changed immediately up until the
first deflate call that consumes any input data.

This avoids unnecessary filling of bytes in the sliding window
buffer when switching from level zero to a non-zero level. This
also provides a consistent indication of deflate having taken
input for a later commit ...

And some cosmetic cleanups.

However this ends up not really being solo, since it has to
include external libraries.

There have been many reports of bugs in the assembler codes
intended to speed up deflate and inflate. They are third-party
contributions in contrib, and so are not supported by the zlib
maintainers.

Also declare z_size_t when compiling solo.

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.

Per request, but its utility is likely to be very limited. See the
comments in zlib.h.

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.