GN_Fab, Free alarm block

IN:
     BHL = pointer to alarm block

OUT, if call successful:
     Fc = 0
OUT, if call failed:
     Fc = 1
     A = error code:
          RC_BAD ($04), bad alarm block, fatal error

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