New module 'memchr2'. * modules/memchr2: New file. * modules/memchr2-tests: Likewise. * lib/memchr2.h: Likewise. * lib/memchr2.c: Likewise, based on memchr.c. * tests/test-memchr2.c: New test. * MODULES.html.sh (String handling): Add memchr2. Signed-off-by: Eric Blake <ebb9@byu.net>