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/config.h

@@ -159,18 +159,22 @@
 
 /*
  *	If COMPRESS is defined, it should contain the full path name of your
- *	'compress' program.  Defining INTERNAL_COMP causes NetHack to do
- *	simpler byte-stream compression internally.  Both COMPRESS and
- *	INTERNAL_COMP create smaller bones/level/save files, but require
- *	additional code and time.  Currently, only UNIX fully implements
- *	COMPRESS; other ports should be able to uncompress save files a
- *	la unixmain.c if so inclined.
+ *	'compress' program.
+ *
  *	If you define COMPRESS, you must also define COMPRESS_EXTENSION
  *	as the extension your compressor appends to filenames after
- *	compression.
+ *	compression. Currently, only UNIX fully implements
+ *	COMPRESS; other ports should be able to uncompress save files a
+ *	la unixmain.c if so inclined.
+ *
+ *	Defining ZLIB_COMP builds in support for zlib compression. If you
+ *	define ZLIB_COMP, you must link with a zlib library.
+ *
+ *	COMPRESS and ZLIB_COMP are mutually exclusive.
+ *
  */
-
-#ifdef UNIX
+ 
+#if defined(UNIX) && !defined(ZLIB_COMP) && !defined(COMPRESS)
 /* path and file name extension for compression program */
 #define COMPRESS "/usr/bin/compress"	/* Lempel-Ziv compression */
 #define COMPRESS_EXTENSION ".Z"		/* compress's extension */
@@ -180,9 +184,41 @@
 #endif
 
 #ifndef COMPRESS
-# define INTERNAL_COMP	/* control use of NetHack's compression routines */
+# define ZLIB_COMP			/* ZLIB for compression */
 #endif
 
+/*
+ *	Internal Compression Options
+ *
+ *	Internal compression options RLECOMP and ZEROCOMP alter the data
+ *	that gets written to the save file by NetHack, in contrast
+ *	to COMPRESS or ZLIB_COMP which compress the entire file after
+ *	the NetHack data is written out.
+ *
+ *	Defining RLECOMP builds in support for internal run-length
+ *	compression of level structures. If RLECOMP support is included
+ *	it can be toggled on/off at runtime via the config file option
+ *	rlecomp.
+ *
+ *	Defining ZEROCOMP builds in support for internal zero-comp
+ *	compression of data. If ZEROCOMP support is included it can still
+ *	be toggled on/off at runtime via the config file option zerocomp.
+ *
+ *	RLECOMP and ZEROCOMP support can be included even if
+ *	COMPRESS or ZLIB_COMP support is included. One reason for doing
+ *	so would be to provide savefile read compatibility with a savefile
+ *	where those options were in effect. With RLECOMP and/or ZEROCOMP
+ *	defined, NetHack can read an rlecomp or zerocomp savefile in, yet
+ *	re-save without them.
+ *
+ *	Using any compression option will create smaller bones/level/save
+ *	files at the cost of additional code and time.
+ */
+
+/* # define INTERNAL_COMP	*/	/* Forces both ZEROCOMP and RLECOMP */
+/* # define ZEROCOMP		*/	/* Support ZEROCOMP compression */
+/* # define RLECOMP		*/	/* Support RLECOMP compression  */
+
 /*
  *	Data librarian.  Defining DLB places most of the support files into
  *	a tar-like file, thus making a neater installation.  See *conf.h