Guidebook update and SEDUCE runtime switch

add SYSCF docs to the Guidebook because it's info needed in a binary distro
Guidebook.tex - also add some missing italics to some "NetHack" occurances
call nethack.org "official"
Guidebook.txt - didn't regenerate cleanly so no diff
add SEDUCE to SYSCF (only partly inspired by the recent email)

include/sys.h

@@ -29,8 +29,15 @@ struct sysopt {
 	int panictrace_glibc;
 # endif
 #endif
+	int seduce;
 };
 E struct sysopt sysopt;
 
+#ifdef SEDUCE
+# define SYSOPT_SEDUCE sysopt.seduce
+#else
+# define SYSOPT_SEDUCE 0
+#endif
+
 #endif /* SYS_H */