Add internal APIs for dealing with time

Add internal APIs for dealing with time

The logging APIs need to be able to generate formatted timestamps
using only async signal safe functions. This rules out using
gmtime/localtime/malloc/gettimeday(!) and much more.

Introduce a new internal API which is async signal safe.

  virTimeMillisNowRaw replacement for gettimeofday. Uses clock_gettime
                      where available, otherwise falls back to the unsafe
                      gettimeofday

  virTimeFieldsNowRaw  replacements for gmtime(), convert a timestamp
  virTimeFieldsThenRaw into a broken out set of fields. No localtime()
                       replacement is provided, because converting to
                       local time is not practical with only async signal
                       safe APIs.

  virTimeStringNowRaw  replacements for strftime() which print a timestamp
  virTimeStringThenRaw into a string, using a pre-determined format, with
                       a fixed size buffer (VIR_TIME_STRING_BUFLEN)

For each of these there is also a version without the Raw postfix
which raises a full libvirt error. These versions are not async
signal safe

* src/Makefile.am, src/util/virtime.c, src/util/virtime.h: New files
* src/libvirt_private.syms: New APis
* configure.ac: Check for clock_gettime in -lrt
* tests/virtimetest.c, tests/Makefile.am: Test new APIs