- Remove some redundant parameters.
- Construct the known_services help string separately (we'll use it
again).
- Exit early if no service has been specified, instead of opening a
frontend with no service and quietly twiddling our thumbs.

No attempt is made here to migrate to argparse.ArgumentParser instead of
the deprecated optparse.OptionParser, though the 'required' argument of
'ArgumentParser.add_argument' would obviate the need for separately
checking whether the '--service' option has been invoked.

At present, a user can grow accustomed to lazily providing lower-case
service names when invoking 'workflows.service' from the command line.
This works fine until you encounter the following situation:
- Known services include "DLSISPyB" and "DLSISPyBPIA";
- user specifies '-s dlsispyb', expecting this to be interpreted as
'DLSISPyB', or else expecting an error;
- not finding a perfect match, the current logic checks whether any
known service names, rendered in lower case, start with 'dlsispyb',
finds two such matches;
- rather than selecting either of these two matches, the program quietly
 continues to open a frontend and doesn't start any service;
- user sees so warning or error, program twiddles its thumbs until
terminated.

New logic:
1. Check for perfect match;
2. Failing that, check for case-insensitive match;
3. Failing that, check for case-sensitive partial matches.  If
ambiguous, exit with error.
4. Failing that, repeat 3 but now case-insensitive.
5. If still no matches, exit with error.