Introduce mi_runto_main

This adds an mi_runto_main routine, very much like the runto_main CLI
counterpart.

Note there's already a mi_run_to_main (extra underscore in "run_to"),
but unlike its intro comment says, that does more than the CLI's
runto_main -- it also starts GDB.  I would like to eliminate that
other one by introducing a mi_clean_restart function instead.  That is
done later in the series.

gdb/testsuite/ChangeLog:

	* lib/mi-support.exp (mi_runto_main): New proc.
	(mi_run_to_main): Use it.
	* gdb.mi/mi-catch-cpp-exceptions.exp: Likewise.
	* gdb.mi/mi-var-cmd.exp: Likewise.
	* gdb.mi/mi-var-invalidate.exp: Likewise.
	* mi-var-list-children-invalid-grandchild.exp: Likewise.
	* gdb.mi/mi2-amd64-entry-value.exp: Likewise.
	* gdb.mi/new-ui-mi-sync.exp: Likewise.
	* gdb.mi/user-selected-context-sync.exp: Likewise.
	* gdb.opt/inline-cmds.exp: Likewise.
	* gdb.python/py-framefilter-mi.exp: Likewise.
	* gdb.python/py-mi.exp: Likewise.

Change-Id: I2e49ca7b0b61cea57c1202e5dfa32417e6a4403d