some doc updates

add a 'distrib' target to the top level Makefile so that the
distribution doc/*.txt files can be created via 'make distrib'
from the top directory.  It utilizes the existing 'distrib'
target in the doc/Makefile.

Have hints/linux.370 specify the correct directory for Linux
man pages of /usr/share/man/man6, so that 'sudo make manpages'
works on that platform,

sys/unix/Makefile.doc

@@ -81,7 +81,7 @@ $(MAKEDEFS)	: ../util/makedefs.c ../include/config.h ../src/mdlib.c \
 	( cd ../util ; make makedefs )
 
 GAME	= nethack
-MANDIR	= /usr/man/man6
+MANDIR	?= /usr/man/man6
 MANEXT	= 6
 
 # manual installation for most BSD-style systems