Fix ancient violation of zlib's API spec.

 Fix ancient violation of zlib's API spec.
 
 contrib/pgcrypto mishandled the case where deflate() does not consume
 all of the offered input on the first try.  It reset the next_in pointer
 to the start of the input instead of leaving it alone, causing the wrong
 data to be fed to the next deflate() call.
 
 This has been broken since pgcrypto was committed.  The reason for the
 lack of complaints seems to be that it's fairly hard to get stock zlib
 to not consume all the input, so long as the output buffer is big enough
 (which it normally would be in pgcrypto's usage; AFAICT the input is
 always going to be packetized into packets no larger than ZIP_OUT_BUF).
 However, IBM's zlibNX implementation for AIX evidently will do it
 in some cases.
 
 I did not add a test case for this, because I couldn't find one that
 would fail with stock zlib.  When we put back the test case for
 bug #16476, that will cover the zlibNX situation well enough.
 
 While here, write deflate()'s second argument as Z_NO_FLUSH per its
 API spec, instead of hard-wiring the value zero.
 
 Per buildfarm results and subsequent investigation.
 
 Discussion: https://postgr.es/m/16476-692ef7b84e5fb893@postgresql.org