site: interpret first 4 bytes of extrainfo as capabilities

Define the first four bytes of the additional data (after the nul in
site names in MSG1..4) in the sender's name field to be a capability
bitmask.  Currently no capability flags are defined.

To support this, replace extrainfo and its _len in struct parsedname
with a struct buffer_if.

We record the capabilities from MSG1 or MSG3, and check that they
haven't changed when processing MSG2..4, as applicable.  (Because
older secnets don't cope with flags in MSG1, we have to define two
kinds of capability flag: ones expected to be in MSG1, and ones which
can appear later in the exchange.)

The support for "Early" capabilities is intended to be used in the
future to support algorithm agility in key exchange.

We also advertise our own capabilities (currently, all-bits-zero) in
our own messages.

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