MEDIUM: xprt: Add a "get_capability" method.

MEDIUM: xprt: Add a "get_capability" method.

Add a new method to xprts, get_capability, that can be used to query if
an xprt supports something or not.
The first capability implemented is XPRT_CAN_SPLICE, to know if the xprt
will be able to use splicing for the provided connection.
The possible answers are XPRT_CONN_CAN_NOT_SPLICE, which indicates
splicing will never be possible for that connection,
XPRT_CONN_COULD_SPLICE, which indicates that splicing is not usable
right now, but may be in the future, and XPRT_CONN_CAN_SPLICE, that
means we can splice right away.