Cleanup etags.c to use locale-independent code

Cleanup etags.c to use locale-independent code

Although this doesn't alter behavior (as etags doesn't use
setlocale), the new version is more clearly locale-independent and
the executable is a bit smaller on my platform.
* lib-src/etags.c: Include <limits.h>, for UCHAR_MAX.
Include <c-ctype.h> instead of <ctype.h>.
(CHARS, CHAR, init, _wht, _nin, _itk, _btk, _etk, white, nonam, endtk)
(begtk, midtk):
Remove; no longer needed.
(iswhite, ISALNUM, ISALPHA, ISDIGIT, ISLOWER, lowcase): Remove.
All callers changed to use c_isspace, c_isalnum, c_isalpha, c_isdigit,
c_islower, c_tolower, respectively.
(notinname, begtoken, intoken, endtoken): Rewrite as functions
instead of macros, and initialize the tables at compile-time
rather than at run-time.