`scdm` now sources `xprofile` before starting X.
Generalized `scdmrc`.

Not everyone has `/etc/X11/start-session.sh` so I removed it from the
default `scdmrc`.
Instead, `scdm` now sources `xprofile` itself
(see commit 1089465f04202535d4d5c4a331348860dfa2d508).