suexec: Support use of setgid/setuid capability bits on Linux, a

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.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1342065 13f79535-47bb-0310-9956-ffa450edef68