Get rid of set_default_file_name()

A global variable protected by a mutex is not actually required to get
the same functionality: the default value can be simply passed to
parse_file_args(), and propagated where needed.  This makes the design
properly thread-safe, allows separation of file type handling from file
argument handling (not done here), and simplifies the logic by removing
confusing bCanNotOverride parameters (replaced by passing the default
file name only where it belongs).

Also, clean up includes in filenm.*.

Change-Id: I831576963b40f9e0a8bbaa741c80c3e59ec6f85c