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

configure.ac

@@ -147,6 +147,16 @@ LIBS="$LIBS $LIB_PTHREAD $LIBMULTITHREAD"
 AC_CHECK_FUNCS([pthread_mutexattr_init])
 LIBS=$old_libs
 
+old_LIBS=$LIBS
+RT_LIBS=
+LIBS="$LIBS $LIB_PTHREAD -lrt"
+AC_CHECK_FUNC([clock_gettime],[
+     AC_DEFINE([HAVE_CLOCK_GETTIME],[],[Defined if clock_gettime() exists in librt.so])
+     RT_LIBS=-lrt
+])
+LIBS=$old_libs
+AC_SUBST(RT_LIBS)
+
 dnl Availability of various common headers (non-fatal if missing).
 AC_CHECK_HEADERS([pwd.h paths.h regex.h sys/un.h \
   sys/poll.h syslog.h mntent.h net/ethernet.h linux/magic.h \