gdb: Re-enable stdin for all UIs from start_event_loop

gdb: Re-enable stdin for all UIs from start_event_loop

If we catch an exception in start_event_loop's call to
gdb_do_one_event, then it is possible that the current_ui has changed
since we called async_disable_stdin.  If that's the case then calling
async_enable_stdin will be called on the wrong UI.

To solve this problem we wrap the call to async_enable_stdin with
SWITCH_THRU_ALL_UIS, this causes us to try and re-enable stdin for all
UIs, which will catch any for which we called async_disable_stdin.

gdb/ChangeLog:

	* event-loop.c (start_event_loop): Wrap async_enable_stdin with
	SWITCH_THRU_ALL_UIS.

gdb/testsuite/ChangeLog:

	* gdb.server/multi-ui-errors.c: New file.
	* gdb.server/multi-ui-errors.exp: New file.

Change-Id: I1e18deff2e6f4e17f7a13adce3553eb001cad93b