Make uN_strstr functions O(n) worst-case.

Make uN_strstr functions O(n) worst-case.

* lib/unistr/u-strstr.h (FUNC): In the 8-bit case, use strstr. In the
16-bit and 32-bit unit cases, use the unibyte algorithm from
lib/mbsstr.c.
* lib/unistr/u8-strstr.c: Include <string.h>.
(UNIT_IS_UINT8_T): New macro.
* lib/unistr/u16-strstr.c: Include malloca.h and str-kmp.h.
(U_STRLEN, U_STRNLEN): New macros.
* lib/unistr/u32-strstr.c: Include malloca.h and str-kmp.h.
(U_STRLEN, U_STRNLEN): New macros.
* modules/unistr/u8-strstr (Depends-on): Add strstr.
(configure.ac): Update required libunistring version.
* modules/unistr/u16-strstr (Files): Add lib/str-kmp.h.
(Depends-on): Add unistr/u16-strlen, unistr/u16-strnlen, stdbool,
malloca.
(configure.ac): Update required libunistring version.
* modules/unistr/u32-strstr (Files): Add lib/str-kmp.h.
(Depends-on): Add unistr/u32-strlen, unistr/u32-strnlen, stdbool,
malloca.
(configure.ac): Update required libunistring version.