Map EPROTONOSUPPORT, ESOCKTNOSUPPORT, EPFNOSUPPORT and EAFNOSUPPORT to
G_IO_ERROR_NOT_SUPPORTED in g_io_error_from_errno(). (GSocket's
socket_io_error_from_errno() already did this with the corresponding
Winsock errors.)

Also map EOPNOTSUPP, which on Linux is the same as ENOTSUP, but may
not be on other platforms.

Also, rewrite the EAGAIN/EWOULDBLOCK section to use the simpler idiom
used by EEXIST/ENOTEMPTY and (now) ENOTSUP/EOPNOTSUPP.