<bsd.progs.mk>: Allow using SRCS for common sources

Instead of having to do:

PROGS= program1 program2
SRCS.program1= program1.c common.c
SRCS.program2= program2.c common.c

This can now be done instead:

PROGS= program1 program2
SRCS= common.c
SRCS.program1= program1.c
SRCS.program2= program2.c

This shouldn't effect any existing projects using <bsd.progs.mk>.

Reviewed by: imp, sjg
Pull Request: https://github.com/freebsd/freebsd-src/pull/766