glibc includes a memmem() function which, by analogy with strstr()
searches for a byte sequence within a larger byte sequence.  The function
isn't standard, however, so other C libraries may not include it.

This adds a trivial module providing the memmem() function, if the C
library doesn't already do so.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>