OS_Gth, Allocate tri-handle

IN:
     A = tri-handle subtype
           TH_FILT ($F0), allocate filter handle
           TH_WMG ($F1), allocate wildcard manager handle
           TH_ALM ($F2), allocate alarm handle
     B = Bank where code is to be called
     H = High byte of adress (Low byte is given by the reason code)
     L = Segment where to bind bank

OUT, if call successful:
     Fc = 0
     IX = tri-handle
OUT, if call failed:
     Fc = 1
          A = RC_ROOM, failed to allocate tri-handle

Registers changed after return:
     ..BCDEHL/..IY same
     AF....../IX.. different