Implement wall clock time limits for shell commands

* Use the "timeout" command to implement wall clock time limits.
* Write a message to stderr when the time limit is exceeded, for
consistency with ulimit
* Configure with $wgMaxShellWallClockTime
* If "time" is specified in $limits, use that as the wall clock limit in
preference to the global variable, to support legacy callers which
usually want to disable the limits.
* Remove the requirement for ulimit5.sh to be executable since this is
not necessary when you specify the interpreter. Should avoid subtle
problems when the executable bit is accidentally stripped.
* Move the interpretation of 0/-1/false limits to the shell script, so
that e.g. wall clock time can be limited even if memory is unlimited.

Change-Id: Id9f2fea7c3b027565bdc33b88a1a7d4cdfe43a6c