core: Fall back to CLOCK_MONOTONIC if CLOCK_BOOTTIME unsupported

It was added fairly recently (2.6.39), this breaks run (and test run in mock)
on RHEL-6 with 2.6.32.
libnm: Ignore NoReply errors when NM has vanished from the bus

This fixes the /libnm/client-nm-running test failure when a race condition is hit:

test-nm-client:10350): libnm-WARNING **: updated_properties:
	error reading NMRemoteSettings properties:
	GDBus.Error:org.freedesktop.DBus.Error.NoReply:
	Message did not receive a reply (timeout by message bus)

What actually happens is that nm_running_changed_cb() calls GetAll() for a
NMRemoteSettings object when NM appears on the bus. If it disappears shortly
afterwards, another nm_running_changed_cb() is called which suppresses further
object updates, but the original GetAll() might not have finished yet and DBus
will generate a NoReply() response for it. We ought to ignore it.
tests: Fix /general/nm_utils_kill_child race

/general/nm_utils_kill_child: **
GLib:ERROR:test-general-with-expect.c:105:test_nm_utils_kill_child_sync_do: Did not see expected message NetworkManager-DEBUG: *kill child process 'test-s-1-1' (*): waiting up to 500 milliseconds for process to terminate normally after sending SIGTERM (15)...
Aborted

The first test case assumes the child does not go away immediately after being
delivered a TERM signal. Add some delay to its teardown code path, so that NM
will set up the timeout the test expects.
contrib/rpm: Run make check
contrib/rpm: Use bconds