sd-varlink: hook up fd passing control with SO_PASSRIGHTS

sd-varlink: hook up fd passing control with SO_PASSRIGHTS

This is a tricky one, because we effectively turn fd passing input
toggle into a tristate: unset, disabled, and enabled; whereas unset
and disabled were identical previously. *Unset* state silently
ignores SCM_RIGHTS passed by invoking recv() instead of recvmsg(),
and for disabled we now disable SO_PASSRIGHTS completely.

The plot thickens when it comes to the server, since we want to
turn off the SO_PASSRIGHTS already on the listening socket so that
there's no race between accept() and recvmsg() wrt SO_PASSRIGHTS state.
However, if we do this unconditionally, the existing use case of
creating a custom connection callback and enabling fd passing there
would be broken.

Hence, let's introduce a new flag,
SD_VARLINK_SERVER_FD_PASSING_INPUT_STRICT, which when set
ties the enablement of fd passing to SO_PASSRIGHTS in server,
and set it for all our varlink servers.

Refer to the previous commit for the rationale behind return value
change in sd_varlink_set_allow_fd_passing_input().