Adjust helptext in m4/systemd.m4

Use AS_HELP_STRING for proper formating, mention both options take a
directory, mention the default value, quote the arguments in brackets.

Please rerun autogen.sh after applying this patch.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Use C xenstored with --disable-ocamltools

When xen is configured with --disable-ocamltools then oxenstored will
still be the default if all required ocaml devel packages are installed.
The reason is that xenstored.m4 is included before the ocamltools check.
And xenstored.m4 does not use the result of --disable-ocamltools.

Rearrange the code to check first for the presence of ocaml, then check
--disable-ocamltools and finally decide what sort of xenstored should be
the default.

Please rerun autogen.sh after applying this patch.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>