Rewrite memmem to guarantee linear complexity without malloc.

Rewrite memmem to guarantee linear complexity without malloc.

* lib/memmem.c (memmem): Use Two-Way rather than
Knuth-Morris-Pratt, to allow O(1) space usage.
(critical_factorization, two_way_short_needle)
(two_way_long_needle): New functions.
(knuth_morris_pratt): Delete.
* modules/memmem (Depends-on): No longer need malloca or stdbool.
Add stdint.
* tests/test-memmem.c (main): Add tests for periodic needle and
sublinear performance.
* doc/functions/memmem.texi (memmem): Document other deficiencies
in cygwin and older glibc.

Signed-off-by: Eric Blake <ebb9@byu.net>