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

MFC after: 1 week
Reviewed by: asomers
Phabric: D1130
Sponsored by: EMC / Isilon Storage Division
Fix build race in Makefile.inc1 after r274226 by adding lib/libm__L dependency
for gnu/lib/libdialog

Sponsored by: EMC / Isilon Storage Division