share/mk: make *ADD and *FLAGS descriptions common

Move ACFLAGS, AFLAGS, CFLAGS, CXXFLAGS, DPADD, LDADD, LDFLAGS, and
LIBADD descriptions into the common section since they aren't limited
to bsd.progs.mk. Mostly just relocated, but updated the DPADD text to
reflect its relocation.

Reviewed by: jhb
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D50479