lib/inet_proto: Review inet_proto_{a2n,n2a}()

lib/inet_proto: Review inet_proto_{a2n,n2a}()

The original intent was to make sure strings written by those functions
are NUL-terminated at all times, though it was suggested to get rid of
the 15 char protocol name limit as well which this patch accomplishes.

In addition to that, simplify inet_proto_a2n() a bit: Use the error
checking in get_u8() to find out whether passed 'buf' contains a valid
decimal number instead of checking the first character's value manually.

Signed-off-by: Phil Sutter <phil@nwl.cc>