pppd: Refactor setjmp/longjmp with pipe pair in event wait loop

setjmp/longjmp isn't supported by all compilers.
Having a pipe pair to wake an event wait loop from within a signal handler
is rather portable and common enough.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>