Allow stack traces to be included with log messages

Sometimes it is useful to see the callpath for log messages.
This change enhances the log filter syntax so that stack traces
can be show by setting '1:+NAME' instead of '1:NAME'.

This results in output like:

2012-05-09 14:18:45.136+0000: 13314: debug : virInitialize:414 : register drivers
/home/berrange/src/virt/libvirt/src/.libs/libvirt.so.0(virInitialize+0xd6)[0x7f89188ebe86]
/home/berrange/src/virt/libvirt/tools/.libs/lt-virsh[0x431921]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x3a21e21735]
/home/berrange/src/virt/libvirt/tools/.libs/lt-virsh[0x40a279]

2012-05-09 14:18:45.136+0000: 13314: debug : virRegisterDriver:775 : driver=0x7f8918d02760 name=Test
/home/berrange/src/virt/libvirt/src/.libs/libvirt.so.0(virRegisterDriver+0x6b)[0x7f89188ec717]
/home/berrange/src/virt/libvirt/src/.libs/libvirt.so.0(+0x11b3ad)[0x7f891891e3ad]
/home/berrange/src/virt/libvirt/src/.libs/libvirt.so.0(virInitialize+0xf3)[0x7f89188ebea3]
/home/berrange/src/virt/libvirt/tools/.libs/lt-virsh[0x431921]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x3a21e21735]
/home/berrange/src/virt/libvirt/tools/.libs/lt-virsh[0x40a279]

* docs/logging.html.in: Document new syntax
* configure.ac: Check for execinfo.h
* src/util/logging.c, src/util/logging.h: Add support for
  stack traces
* tests/testutils.c: Adapt to API change

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

configure.ac

@@ -158,7 +158,7 @@ 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 \
   sys/un.h sys/syscall.h netinet/tcp.h ifaddrs.h libtasn1.h \
-  net/if.h])
+  net/if.h execinfo.h])
 
 AC_MSG_CHECKING([for struct ifreq in net/if.h])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM(