tools: add configure --with-dumpdir=DIR option

tools: add configure --with-dumpdir=DIR option

The current base directory /var/xen/dump for domU dumps will be patched
to /var/lib/xen/dump by most distros to follow FHS.

This change does three things:
 - change the default from /var/xen/dump to /var/lib/xen/dump
 - provide a configure option to avoid patching the source.
 - update docs to refer to the new default location

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- s,/var,LOCALSTATEDIR, in help test, ran autogen.sh ]