Mobile sites: Require specification of whether we think we are mobile

We introduce a new config option "local-mobile" which must match our
peers' idea of whether we are mobile.  This is cross-checked with the
"site" entry for our own site, if there is one, to detect mistakes.

We do not transfer this data in the protocol because we don't want to
break compatibility with older secnets which do not understand mobile
peers at all.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>