[Bug-gnulib] <string.h> replacements update

[Bruno Haible]

Hi,

I'm committing these changes. Hope most of you agree.

        * strcspn.c: Include <stddef.h>.
        (strcspn): Use ANSI C function declaration. Change return type to
        size_t. Use NULL.

size_t is what the return type should be. NULL instead of 0 is for
readability and to minimize difference with glibc. <stddef.h> is
needed for size_t and NULL.

        * strpbrk.c: Minimize diffs to glibc. Include <stddef.h>.
        (strpbrk): Use NULL.

NULL instead of 0 is for readability and to minimize difference with
glibc. <stddef.h> is needed for NULL.

        * strstr.c: Don't include <sys/types.h>.

<sys/types.h> does not exist on Woe32, and is not needed by this file
anyway.

        * strpbrk.h (PARAMS): Remove macro.
        (strpbrk): Use ANSI C function declaration.
        * strstr.h (PARAMS): Remove macro.
        (strstr): Use ANSI C function declarations.

Move to ANSI C. Btw, regex.c in coreutils is already assuming ANSI C
for 2.5 years.

Bruno