libsysdecode: support rfork(RFSPAWN)

Match the "U" in RFSPAWN's value (1U<<31) (and "u" for completeness).
Reject kernel-only RFPPWAIT to avoid a duplicate entry.

Reviewed by: imp, markj
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D45597