waitpid: improve safety

waitpid: improve safety

Based on a report by Coverity.  waitpid() can leak resources if it
fails with EINTR, so it should never be used without checking return
status.  But we already have a helper function that does that, so
use it in more places.

* src/lxc/lxc_container.c (lxcContainerAvailable): Use safer
virWaitPid.
* daemon/libvirtd.c (daemonForkIntoBackground): Likewise.
* tests/testutils.c (virtTestCaptureProgramOutput, virtTestMain):
Likewise.
* src/libvirt.c (virConnectAuthGainPolkit): Simplify with virCommand.