lib: allow explicit recpars on command line

The --recpars command line option has been extended
to allow recipe parameters to be given explicitly,
as well being as read from one file.  This change
has been made in preparation for tiling in the JSA.

From: Tim Jenness <tim.jenness@gmail.com>
Date: Mon, 2 Sep 2013 13:47:51 -0700

[...] extend the `--recpars` syntax in a similar manner to
the Starlink GROUP syntax, allowing explicit override parameters
on the command line as well as a file name.

    --recpars="tilenum=456356,recpars-M95AC34.ini"