Fixed permissions check in open_registry().

We were checking for the presence of KEY_WRITE in the SAM to decide
whether to call RegCreateKeyEx() or RegOpenKeyEx().  KEY_WRITE is an
alias for STANDARD_RIGHTS_WRITE | KEY_SET_VALUE | KEY_CREATE_SUB_KEY so
the function was incorrectly calling RegCreateKeyEx() even when
RegOpenKeyEx() would be more appropriate.