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,
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