Support custom default extension for FileNameOption

Add FileNameOption.defaultType() to allow changing the default extension
used for file types that accept multiple extensions.  About 1% of the
existing t_filenm declarations seem to use such a feature.

Make parse_common_args() deduce the value for this from the value of
t_filenm.fn, if it contains an extension.

Extend unit tests to cover these cases.

Change-Id: Idf8d1c6b70b88d58bbf12398bd16f57412eab065