ref:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/strcasecmp_l.html

This patch introduces the `strcasecmp_l` function. At present, the
locale parameter is ignored, making it a stub implementation. This is
consistent with how other locale-related functions, such as `islower_l`,
are treated in our codebase as well as in
[musl](https://github.com/bminor/musl/blob/master/src/string/strcasecmp.c)
and
[bionic](https://cs.android.com/android/platform/superproject/main/+/main:bionic/libc/bionic/strings_l.cpp).

---------

Co-authored-by: Michael Jones <michaelrj@google.com>

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This patch adds the `strings/ffs` function.
ref: https://pubs.opengroup.org/onlinepubs/9799919799/functions/ffs.html
Closes: #122054.

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

Since `index_test.cpp` and `rindex_test.cpp` now reside in /strings,
adjust some header paths accordingly.

Link: #118899

docgen relies on the convention that we have a file foo.cpp in
libc/src/\<header\>/. Because the above functions weren't in libc/src/strings/
but rather libc/src/string/, docgen could not find that we had implemented
these.

Rather than add special carve outs to docgen, let's fix up our sources for
these 7 functions to stick with the existing conventions the rest of the
codebase follows.

Link: #118860
Fixes: #118875