Add syntax to force selection string matching mode.

Add syntax to force selection string matching mode.

The selection syntax for string keyword matching no longer depends on
whether regular expression support is available.  Instead, an error is
now given if the string looks like a regexp (the logic for the deduction
is not changed), but regexp support is not available.  Added syntax to
force the string matching to use either literal, wildcard, or regexp
matching.

This change allows removing a few more direct prints to stderr (related
to #655).

Closes #938.

Change-Id: I7b998050c8b00b5f1229ed23a0a15685c514010f