bsd.man.mk: Add a MANSRC.{TARGET} variable

This can be used to specify an alternate source file name for an
individual manpage. For example, the following snippet:

MAN=   foo::bar.3
MANSRC.foo::bar.3= foo__bar.3

Will install a foo::bar.3 manpage but use foo__bar.3 as the source
input to MANFILTER, etc.

Suggested by: ivy
Reviewed by: ivy
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D51793