zlib support; also internal compression changes

o Add support for zlib compression via ZLIB_COMP in config.h (ZLIB_COMP
  and COMPRESS are mutually exclusive).
o rlecomp and zerocomp are run time options available if RLECOMP and
  ZEROCOMP are defined, but not turned on by default if either COMPRESS
  or ZLIB_COMP are defined.
o Add information to the save file about internal compression options
  used when writing the save file, particularly rlecomp and zerocomp
  support.
o Automatically adjust rlecomp and zerocomp (if support compiled in)
  when reading in an existing savefile that was saved with those options
  turned on.  Still allows writing out of savefile in preferred format.
o In order to support zlib and not conflict with compress and uncompress
  routines there, the NetHack internal functions were changed to
  nh_uncompress and nh_compress as done in the zlib contribution received
  in 1999 from <Someone>.

I tagged the sources NETHACK_3_5_0_PREZLIB prior to applying these
changes.

include/flag.h

@@ -164,6 +164,8 @@ struct instance_flags {
 	boolean  menu_tab_sep;	/* Use tabs to separate option menu fields */
 	boolean  menu_requested; /* Flag for overloaded use of 'm' prefix
 				  * on some non-move commands */
+	boolean  zerocomp;	/* write zero-compressed save files */
+	boolean  rlecomp;	/* run-length comp of levels when writing savefile */
 	uchar num_pad_mode;
 	int	menu_headings;	/* ATR for menu headings */
 	int      purge_monsters;	/* # of dead monsters still on fmon list */