new RTS flag: -V to modify the resolution of the RTS timer

new RTS flag: -V to modify the resolution of the RTS timer
Fixed version of an old patch by Simon Marlow. His description read:
 Also, now an arbitrarily short context switch interval may now be
 specified, as we increase the RTS ticker's resolution to match the
 requested context switch interval.  This also applies to +RTS -i (heap
 profiling) and +RTS -I (the idle GC timer).  +RTS -V is actually only
 required for increasing the resolution of the profile timer.