gdb/mi: make current_token a field of mi_interp

gdb/mi: make current_token a field of mi_interp

Following the commit f818c32ba459 ("gdb/mi: fix ^running record with
multiple MI interpreters"), I thought it would make sense to make
current_token a field of mi_interp.  This variable contains the token of
the currently handled MI command, like the 222 in:

    222-exec-continue

I didn't find any bug related to that, it's just a "that seems nicer"
cleanup, since the current token is a fundamentally per-interp thing.

mi_execute_command needs a check similar to what we already have in
mi_cmd_gdb_exit: when invoked from Python's gdb.execute_mi, the current
interpreter is not an mi_interp.  When using the Python gdb.execute_mi
function, there is no such concept of token, so we can just skip that.

There should be no user-visible change.

Change-Id: Ib52b3c0cba4b7c9d805b432c809692a86e4945ad
Approved-By: Tom Tromey <tom@tromey.com>