Added AIX Crontab support

On AIX the crontab command has the following format :-

crontab [ -e [UserName] | -l [UserName] | -r [UserName] | -v [UserName] | File ]

This is different from the Linux crontab [-u user] ... format.

While AIX could be added to the Solaris options (for both) it makes sense to me to use the native format for crontab -l user to grab the file.