Remove unnecessary extensions in t_filenm defaults

Several tools had an extension specified for the default file name in
t_filenm, but that extension matched either the only accepted extension
for the file type, or to the default extension for that type.

To keep the code more uniform, remove those unnecessary declarations.
This leaves <10 instances where t_filenm specifies a non-default
extension for the default file name (which doesn't currently work).
Probably some of those could also be removed, but at least a few are
justified, so a subsequent commit will add support for that.

Change-Id: I28053b6b407783b515144aad5351f27bbc2b6ba7
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