Make getfdproto return enum instead of string

Make getfdproto return enum instead of string

Introduce a new enum type sock_proto and use it instead of strings for socket
protocols identification.

* defs.h (sock_proto): New enum.
(get_proto_by_name): New function.
* socketutils.c (protocols): New static table.
(print_sockaddr_by_inode): Use it.  Change type of "proto" argument
to sock_proto.
(get_proto_by_name): New function.
* util.c (getfdproto): Use it.  Change return type to sock_proto.
(printfd): Update.