Globally replace 'global_sam_sid' with get_global_sam_sid(), a self

Globally replace 'global_sam_sid' with get_global_sam_sid(), a self
initialising function.  This patch thanks to the work of
"Stefan (metze) Metzmacher" <metze@metzemix.de>

This is partly to enable the transition to SIDs in the the passdb.

Andrew Bartlett
(This used to be commit 96afea638e15d4cbadc57023a511094a770c6adc)