gdb/testsuite: extend the use of mi_clean_restart

gdb/testsuite: extend the use of mi_clean_restart

The mi_clean_restart proc calls the mi_gdb_start proc passing no
arguments.

In this commit I add an extra (optional) argument to the
mi_clean_restart proc, and pass this through to mi_gdb_start.

The benefit of this is that we can now use mi_clean_restart when we
also want to pass the 'separate-mi-tty' or 'separate-inferior-tty'
flags to mi_gdb_start, and avoids having to otherwise duplicate the
contents of mi_clean_restart in different tests.

I've updated the obvious places where this new functionality can be
used, and I'm seeing no test regressions.

Reviewed-By: Pedro Alves <pedro@palves.net>