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
This way qrexec-client-vm will have much more information, at least:
- will know whether the service call was accepted or refused
- potentially will know remote process exit code
This commit implements the first point - the local process will not be
started if service call was refused.
This process should be started from user session (most likely
qubes-session). New processes (of that user) will be created as
children of that session making logind and such crap happy. This should
also solve problems with EOF transmission (no additional "su" process)
and prevent loading all the environment multiple times.
