r325740 moved dhclient to the cap_syslog service, while r341692
changed cap_syslog to inherit stdio descriptors from its parent process.
The broke the ability to capture dhclient's foreground output, as the
cap_syslog service now holds the original stdio descriptors open after
dhclient daemonizes. Since syslog(3) only requires stdio access for the
LOG_PERROR case (which dhclient doesn't use), instead revert r341692
and in the LOG_PERROR case supply the parent's stderr descriptor as
an argument within the implementation of cap_openlog().