xenstore size limits

xenstore size limits

 * Documents the existing 4kby size limit on xenstore message payloads
 * Causes xs.c in libxenstore to fail locally rather than violating
   said limit (which is good because xenstored kills the client
   connection if it's exceeded).
 * Introduces some limits on path lengths in xenstored.  I trust
   no-one is using path lengths >2kby.  This is good because currently
   a domain client can create a 4kby relative path that the dom0 tools
   cannot access since they'd have to specify the somewhat longer
   absolute path.
 * Removes uses of the host's PATH_MAX (!)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>