Add support for --start option in -exec-run GDB/MI command.

gdb/ChangeLog:

        * mi/mi-main.c (run_one_inferior): Add function description.
        Make ARG a pointer to an integer whose value determines whether
        we should "run" or "start" the program.
        (mi_cmd_exec_run): Add handling of the "--start" option.
        Reject all other command-line options.
        * NEWS: Add entry for "-exec-run"'s new "--start" option.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Program Execution): Document "-exec-run"'s
        new "--start" option.

gdb/testsuite/ChangeLog:

        * gdb.mi/mi-start.c, gdb.mi/mi-start.exp: New files.