MFC r315053-r315054, r315056

MFC r315053-r315054, r315056

r315053:
Add share/man if it exists to the MANPATH

localbase is not consistent with base for manpages:
/usr/local/man vs /usr/share/man adding share/man allows to fix that
inconsistency and would permit to remove tons of patches/modifications in the
ports tree

r315054:
Extend functionality MANPATH in man(1) to followup with apropos(1) from
mandoc.

If MANPATH begins with a colon, it is appended to the default list; if it ends
with a colon, it is prepended to the default list; or if it contains two
adjacent colons, the standard search path is inserted between the colons.  If
none of these conditions are met, it overrides the standard search path.

Import the MANPATH description from mandoc into the man(1) man page

Reported by:	kargl
MFC after:	1 week

r315056:
Remove the warning when MANPATH is set in the environment

The MANPATH environment variable behaviour is documented properly in the manpage
and it now has extended to new feature that allows to make MANPATH env variable
extending the default search path rather than overwriting it making the warning
painful

Reported by:	kargl
MFC after:	1 week