procctl.2: Fix names of PROC_LOGSIGEXIT_CTL constants

The headers contain constants that start with PROC_LOGSIGEXIT_CTL_*, but the
man page elided the _CTL portion.

Fixes: dabf006 ("Add per-process flag to disable logsigexit")
Reviewed by: imp, jlduran
Pull Request: https://github.com/freebsd/freebsd-src/pull/1610