Fallback to home dir if initial working dir is inaccessible

In cases where the initial working is inaccessible by the current
user, we should fallback to their home directory.

This can easily be triggered by changing the working dir to an
otherwise inaccessible dir for the current user (e.g. /etc/sudoers.d
inside a root shell), followed by opening a new session.

BUG: 470262
CCBUG: 469249
CCBUG: 475116
(cherry picked from commit e4840a2e)