Add some comments for variables and targets.
Include <bsd.obj.mk>, remove targets obj, clean, cleandir.
Replace ${MAN*} with ${DOC*} variables.
Use a .for loop for undefined targets
FORMATS can be defined as an empty string to suppress generation or
installation of any files. Previously setting it to "null" had that
effects.
Now uses MANOWN, MANGRP and MANMODE for installation instead of BINOWN,
BINGRP and BINMODE.
