Instead of calling 'su' to switch the user, use own implementation of
this. Thanks to PAM it's pretty simple. The main reason is to have
control over process waiting for session termination (to call
pam_close_sesion/pam_end). Especially we don't want it to keep std* fds
open, which would prevent qrexec-agent from receiving EOF when one of
them will be closed.
Also, this will preserve QREXEC_AGENT_PID environment variable.
FixesQubesOS/qubes-issues#2851
