emcsh: support rapid_override

add command emc_rapid_override (for tcl package Linuxcnc)
to complement existing command emc_feed_override

usage example (while running linuxcnc):
   $ tclsh
   % package require Linuxcnc
   % emc_init inifilename
   % emc_rapid_override

Signed-off-by: Dewey Garrett <dgarrett@panix.com>