maint: use to_uchar function rather than explicit casts

* src/system.h (to_uchar): Define function.
* src/kwsearch.c (Fexecute): Use to_uchar twice in place of casts.
* src/dfasearch.c (EGexecute): Likewise.
* src/main.c (prepend_args): Likewise.
* src/kwset.c (U): Define in terms of to_uchar.
* src/dfa.c (match_mb_charset): Use to_uchar, not an explicit cast.
speed up mb-boundary-detection after each preliminary match

After each kwsexec or dfaexec match, we must determine whether
the tentative match falls in the middle of a multi-byte character.
That is what our is_mb_middle function does, but it was expensive,
even when most input consisted of single-byte characters.  The main
cost was for each call to mbrlen.  This change constructs and uses
a cache of the lengths returned by mbrlen for unibyte values.
The largest speed-up (3x to 7x, CPU-dependent) is when most
lines contain a match, yet few are printed, e.g., when using
grep -v common-pattern ... to filter out all but a few lines.

* src/search.h (build_mbclen_cache): Declare it.
* src/main.c: Include "search.h".
[MBS_SUPPORT] (main): Call build_mbclen_cache in a multibyte locale.
* src/searchutils.c [HAVE_LANGINFO_CODESET]: Include <langinfo.h>.
(mbclen_cache): New global.
(build_mbclen_cache): New function.
(is_mb_middle) [HAVE_LANGINFO_CODESET]: Use it.
* NEWS (Improvements): Mention it.