Exclude signal-related symbols on VMS without home-grown signalling.

Exclude signal-related symbols on VMS without home-grown signalling.

config.h defines KILL_BY_SIGPRC when we are using our own signal
handling functions based on the sys$sigprc system service.  But
when *not* doing that, and since using makedef.pl to define our
exportable symbols, we have not properly excluded the symbols
involved in the home-grown implementation.  It's not a common
configuration, but since it's configurable, it should work, and
now it will.