Add memmem-simple module. * m4/memmem.m4 (gl_FUNC_MEMMEM_SIMPLE): New macro. (gl_FUNC_MEMMEM): Separate performance from presence checks. * modules/memmem-simple: New file. * modules/memmem (Description): Tweak. * MODULES.html.sh (string handling): Mention it. * doc/functions/memmem.texi (memmem): Distinguish which flaws are addressed by memmem-simple. * NEWS: Document the difference. Signed-off-by: Eric Blake <ebb9@byu.net>