main/arpwatch: fix build problem, add doc subpackage

Without the chmod the following occur in the prepare phase:

>>> arpwatch: 21_all_arpwatch-2.1a15-pid-filename.patch
File arpwatch.8 is read-only; trying to patch anyway
patching file arpwatch.8
File arpwatch.h is read-only; trying to patch anyway
patching file arpwatch.h
File arpwatch.c is read-only; trying to patch anyway
patching file arpwatch.c
>>> arpwatch: 22_alpine_bihourly_script.patch
File bihourly.sh is read-only; trying to patch anyway
patching file bihourly.sh
>>> arpwatch: Updating ./config.sub
cp: cannot create regular file './config.sub': Permission denied
>>> ERROR: arpwatch: prepare failed

Some lint problems were addressed, but more remain as the patches do not have descriptions. I don't fully understand what each patch does, so I did not attempt to address those lint problems.

Finally, the manpages and project docs are now installed in a doc package.