bsd.man.mk guard against empty vars in MLINKS

share/man/man4/Makefile adds a number of
variables to MAN and MLINKS, which are only set for
certain architectures.

The empty variables wreak havoc when := is used.

Add :M*.[1-9] to MLINKS reference for STAGE_LINKS.mlinks
to avoid invalid results.

Reviewed by: stevek