suexec: Add support for logging to syslog as an alternative to a
logfile.

* support/suexec.c (err_output) [AP_LOG_SYSLOG]: Log to syslog.
  (main): Close syslog fd if open, before execv.  Add -V output
  for AP_LOG_SYSLOG.

* configure.in: Add --with-suexec-syslog argument; allow
  --without-suexec-logfile to omit definition of AP_LOG_EXEC.


suexec: Support use of setgid/setuid capability bits on Linux, a
weaker set of privileges than the full setuid/setgid root binary.

* configure.in: Add --enable-suexec-capabilites flag.

* Makefile.in: If configured, use setcap instead of chmod 7555 on
  installed suexec binary.

* modules/arch/unix/mod_unixd.c (unixd_pre_config): Drop test for
  setuid bit if capability bits are used.

* docs/manual/: Add docs.


* docs/manual/suexec.html.en: Update for syslog logging.


* configure.in: Correct handling of --with-suexec-logfile in r1341905,
  thanks to rpluem.


* support/suexec.c: Define ...