memmem(3): empty little string matches the beginning of the big string

This function originated in glibc, and this matches their behaviour
(and NetBSD, OpenBSD, and musl).

An empty big string (arg "l") is handled by the existing
l_len < s_len test.

Reviewed by: bapt, ngie
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2657