Add a small helper for ssh_buffer to ensure that the buffer has a
certain amount of space already preallocated. This can be useful in case
it is known how much data is going to be added to a buffer, to avoid
multiple reallocations.
Make use of it in few places in the library.
Signed-off-by: Pino Toscano <ptoscano@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>