Add gdb.Objfile.username.

Add gdb.Objfile.username.

gdb/ChangeLog:

	* NEWS: Mention gdb.Objfile.username.
	* python/py-objfile.c (objfpy_get_username): New function.
	(objfile_getset): Add "username".

gdb/doc/ChangeLog:

	* python.texi (Objfiles In Python): Document Objfile.username.

gdb/testsuite/ChangeLog:

	* gdb.python/py-objfile.exp: Add tests for objfile.username.
	Add test for objfile.filename, objfile.username after objfile
	has been unloaded.