...so move it from osl/mutex.hxx to its own comphelper/solarmutex.hxx.  It looks
like a newbie mistake that 59e7685d "Create an
abstract interface to be used to implement a SolarMutex" put it here in the
first place.

I do not consider this an incompatible change really, as no external URE client
code should have used SolarMutex anyway.

(Also included some clean up, like removing unused
{Clearable,Resettable}SolarGuard, and spelling out SolarGuard in the few places
it is used.)

Change-Id: I121ffb5b7cefbc19e88b5405e5a85ffc895be852