slave: Remove more dead properties
worker: Properly detect the error code from setting PAM_TTY

Without this, we'll look at the error code for the last set if we have
an empty TTY, rather than the one for the current set.
session: Prevent memory leaks by removing strdup accessors

gdm_session_get_* dup the strings they return, but not a lot of code
frees the resulting strings. It's a lot easier to not dup the return
values and make them const char *, as they won't ever change while
some code holds onto them...