INT 21 - European MSDOS 4.0 - SET SIGNAL HANDLER
        AH = 8Ch
        AL = signal number (see below)
        BL = action (see below)
        DS:DX -> signal handler
Return: CF clear if successful
            AL = previous action
            ES:BX -> previous signal handler
        CF set on error
            AX = error code (01h,invalid SigNumber or Action) (see AH=59h)
Desc:   set the routine which will be invoked on a number of exceptional
          conditions
Note:   all signals will be sent to the most recently installed handler

Values for signal number:
 01h    SIGINTR         Control-C or user defined interrupt key
 08h    SIGTERM         program termination
 09h    SIGPIPE         broken pipe
 0Dh    SIGUSER1        reserved for user definition
 0Eh    SIGUSER2        reserved for user definition

Values for signal action:
 00h    SIG_DFL         terminate process on receipt
 01h    SIG_IGN         ignore signal
 02h    SIG_GET         signal is accepted
 03h    SIG_ERR         sender gets error
 04h    SIG_ACK         acknowledge received signal and clear it, but don't
                        change current setting

Signal handler is called with:
        AL = signal number
        AH = signal argument
Return: RETF, CF set: terminate process
        RETF, CF clear, ZF set: abort any interrupted system call with an error
        RETF, CF clear, ZF clear: restart any interrupted system call
        IRET: restart any interrupted system call
Note:   the signal handler may also perform a nonlocal GOTO by resetting the
          stack pointer and jumping; before doing so, it should dismiss the
          signal by calling this function with BL=04h