Load sessions and greeters from /usr/share/lightdm/sessions and /usr/share/lightdm...

This is to allow new sessions and greeters to be defined that aren't X based.
We will also look in the legacy directories /usr/share/xsessions and /usr/share/xgreeters.
Rename the config options xsessions-directory->sessions-directory and xgreeters-directory->greeters-directory.
Support multiple directories separated by ':'.
Remove the command line --xsessions-dir, --remote-sessions-dir and --xgreeter-dir options.
They were used primarily for the regression tests and are no longer required.

Approved by Robert Ancell, PS Jenkins bot.