Always set QREXEC_AGENT_PID variable, setup SIGUSR1 handler. And do that
before starting child process to avoid race conditions.
Required for QubesOS/qubes-issues#
FixesQubesOS/qubes-issues#1863
This doesn't cover all the cases, because local process could want to
receive that value (currently it cant), but I can't think of any simple,
*compatible* way to pass it there.
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.
Move (qrexec-agent version of) do_exec to qrexec-agent.c, move
handle_handshake to qrexec-agent-data.c (common to all agent binaries).
Fix indentation (tabs -> spaces).
