gdb/python: new Progspace.executable_filename attribute

Add a new Progspace.executable_filename attribute that contains the
path to the executable for this program space, or None if no
executable is set.

The path within this attribute will be set by the "exec-file" and/or
"file" commands.

Accessing this attribute for an invalid program space will raise an
exception.

This new attribute is similar too, but not the same as the existing
gdb.Progspace.filename attribute.  If I could change the past, I'd
change the 'filename' attribute to 'symbol_filename', which is what it
actually represents.  The old attribute will be set by the
'symbol-file' command, while the new attribute is set by the
'exec-file' command.  Obviously the 'file' command sets both of these
attributes.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>