syslogd: Pipes need the CAP_PDGETPID right as well

While here, use caph_rights_limit(), as syslogd already uses
caph_enter().

PR: 298104
Reported by: mi
Fixes: 24816abb8740 ("syslogd: Limit rights on procdescs")
MFC after: 3 days

(cherry picked from commit 87cfe4a62078bc8ff14671fde3af347e7c7e3a26)