Add a usage string and a defaults section that clarifies:
- If no input files are given, f18 reads from stdin
- If no input files are given, f18 dumps the parse tree.
- The default behaviour is to exec F18_FC.
- The fefault F18_FC setting is 'gfortran'
Adds a simple regression test which tests the top and tail of the help
screen and the exit status.
Depends on D84855