bsd build support

Originating from https://github.com/NetHack/NetHack/pull/1519,
there was an issue with the pull request's back-end fork or with
the pull request itself.

The code changes were applied manually instead, with credit to the
pull request's author, instead of being directly merged in via the
pull request.

Contributed by @SirWumpus on GitHub.

Also,
- fixed a bit of conditional code in include/unixconf.h, where the
  #else clause remained out of reach for non-bsd systems, but was
  needed..
- added a disclaimer to the contributed sys/unix/hints/netbsd.500.

include/unixconf.h

@@ -170,7 +170,7 @@
 #ifdef AMS
 #define AMS_MAILBOX "/Mailbox"
 #else
-#if defined(__FreeBSD__) || defined(__OpenBSD__)
+#if defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
 #define DEF_MAILREADER "/usr/bin/mail"
 #else
 #define DEF_MAILREADER "/usr/ucb/Mail"
@@ -331,9 +331,14 @@
 #include <sys/wait.h>
 #endif
 
+#if defined(__NetBSD__)
+#define tparm2(s, x)	tparm(s,x,0,0,0,0,0,0,0,0)
+#else
+#define tparm2(s, x)	tparm(s,x)
+#endif
 
 #if defined(BSD) || defined(ULTRIX)
-#if !defined(DGUX) && !defined(SUNOS4)
+#if !defined(DGUX) && !defined(SUNOS4) && !defined(__NetBSD__)
 #define memcpy(d, s, n) bcopy(s, d, n)
 #define memcmp(s1, s2, n) bcmp(s2, s1, n)
 #endif