Simpler required selection input from a file.

Simpler required selection input from a file.

Now a selection file option (-sf) assigns the contents of the file to
required selection option(s) that haven't yet been assigned if it has no
other context.  Main benefit is that a plain '-sf file.dat' (without any
other selection options specified) assigns the contents of file.dat to
required selections, similarly to how it worked in 4.5.

Resolves the main open issue for #656, error message and help
improvements remain.

Change-Id: I6311b548ab21ecf0f489f2743e2750411f86b3ff