fix false negatives with periodic needles in strstr, wcsstr, and memmem

in cases where the memorized match range from the right factor
exceeded the length of the left factor, it was wrongly treated as a
mismatch rather than a match.

issue reported by Yves Bastide.

include cleanups: remove unused headers and add feature test macros

use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

remove buggy short-string wcsstr implementation; always use twoway

since this interface is rarely used, it's probably best to lean
towards keeping code size down anyway. one-character needles will
still be found immediately by the initial wcschr call anyway.

initial check-in, version 0.5.0