Use PROGS instead of PROG and remove unnecessary SRCS?= assignment

[ngie]

Summary

Use PROGS instead of PROG and remove unnecessary SRCS?= assignment

Using PROG instead of PROGS will in cases of high -j with -DNO_ROOT cause
the PROG to show up more than once as it's handling the SCRIPTS install case
in a recursive manner, separate from the non-recursive case

After the recent batch of commits to bsd.progs.mk to fix behavior with how
variables are defaulted to, explicitly setting SRCS for a PROG is no longer
required

[asomers]

LGTM. Should ATF tests always use PROGS instead of PROG?

[ngie]

In D1130#4, @asomers wrote:

LGTM. Should ATF tests always use PROGS instead of PROG?

Based on my empirical observations, yes. I'll add some notes to the wiki about this and communicate this out to -testing@.

[ngie]

Committed in r274267 -- thanks!