GN_Aab, Allocate alarm block

IN:  -

OUT, if call successful:
     Fc = 0
     BHL = pointer to alarm block


OUT, if call failed:
     Fc = 1
     A = error code
          RC_ROOM ($07), no room to allocate alarm block

Registers changed after return:
     ...CDE../IXIY same
     AFB...HL/.... different