Moved sdb_proto_send/sdb_proto_select to sdb_write/sdb_select.

These functions are not protocol-specific but rather simple wrappers around
system calls.
proto: Add sdb_proto_unmarshal_header().
connection: Use sdb_proto_unmarshal_header.
connection: Use sdb_connection_close to close a connection.
proto: Renamed sdb_proto_get_int to sdb_proto_unmarshal_int.

… to be consistent with the other functions.
utils/strbuf: Added SDB_STRBUF_STR.

This macro returns a tuple describing the buffer content and length. The idea
is that functions which operate on (binary) strings should accept two such
arguments and the macro will make it easier to pass them in.
utils/proto: Let all unmarshal functions accept strings instead of strbufs.

This is more consistent and flexible.
proto: Added sdb_proto_marshal_data.

This function encodes a datum into the wire format.