MEDIUM: Add external check

Add an external check which makes use of an external process to
check the status of a server.

---

v6
* Correct implementation and documentation of arguments to external-check
  command so that they are consistent with both each other and ldirectord's
  external check. The motivation being to allow the same scripts to be used
  with both haproxy and ldirectord.

v5
* Rebase

v4
* Remove stray use of s->check in process_chk()
  The check parameter should be used throughout process_chk()
* Layer 7 timeouts of agent checks should be ignored
* Ensure that argc is never used uninitialised in prepare_external_check()

v3
* Rebase: basically a rewrite of large sections of the code
* Merge with the following patches
  + "external-check: Actually execute command"
  + "Allow selection of of external-check in configuration file"

v2
* If the external command exits normally (WIFEXITED()) is true)
  then set the check's code to the exit status (WEXITSTATUS())
  of the process.
* Treat a timeout is a failure case rather than the test having passed
* Remove duplicate getnameinfo() call in start_checks()
* Remove duplicate assignment of sockaddr argument to getnameinfo(9
  which caused the check port and check addr configuration of
  a server to be ignored.