Rework GMutex and GCond APIs

Rework GMutex and GCond APIs

Do a substantial rework of the GMutex and GCond APIs.

 - remove all of the macro indirection hackery which is no longer needed
   since we dropped support for switchable thread implementations

 - expose the structure types and add G_MUTEX_INIT and G_COND_INIT
   static initialiser macros

 - add g_mutex_init() and g_mutex_clear() for use when embedding GMutex
   into another structure type and do the same for GCond as well

 - avoid infinite recursion hazards by ensuring that neither GCond or
   GMutex ever calls back into any other part of GLib

 - substantially rework the Windows implementation of GCond and GMutex
   to use the SRWLock and CONDITION_VARIABLE APIs present on Windows
   2008/Vista and later, emulating these APIs on XP