Default for prestart|poststart|prestop|poststop commands

I found it convenient to have the pre|post start|stop commands in `/iocage/jails/<uuid>`, just like there is already a default location for the fstab file. Benefits: 
* Scripts get automatically included in zfs send.
* Easy quickstart for simple cases. Just drop a script with the proper name in the jail dir.
* By the file location you automatically see which jail uses that script.

Therefore I modified iocage so it looks for an executable `/iocage/jails/<uuid>/(prestart|poststart|prestop|poststop)` and only uses the configured  value of the corresponding `exec_*` jail property if no such file was found.

What do you think?

P.S.: Just iocage modification, docs not updated yet.