Sync lib/libc/string with FreeBSD:

Sync lib/libc/string with FreeBSD:

-Move legacy functions to strings.h.

-A few WARNS and style cleanup.

-Add ffsl(), fls(), flsl(), ffsll(), flsll().

-Merge index.3 and rindex.3 since they are so similar.  Do the same for
strrchr().

-Add memrchr().

-Remove advertising clause from copyrights.

-Add memmem() which is basically the same as strstr().

-Add stpcpy().

-Add a SECURITY CONSIDERATIONS section to remind people about buffer
overflows, etc.

-Add restrict keyword where required.

-Add strndup().

-Correctly document the return value of strerror() and strerror_r() and
the contents of the returned buffer for unknown error codes.

-Add NLS catalogs support to strerror(), strerror_r() and strsignal().
Controlled by NLS define.

-For strl* files, use a less restrictive copyright (original author
changed it).

-strmode.3: strmode does not return 0.

-Move swab()'s prototype to unistd.h IAW SUS.

-Simplify by removing unneeded local variables and explicit
null termination is wcs* functions.

-Move wcscoll() and wcsxfrm() to string/ and do locale-sensitive
collation for single-byte locales.

-Add wcsdup().

-Remove unneeded includes.

-Reimplement wcsrchr(3) more efficiently, using a single forward scan
(like strrchr(3)) instead of scanning forwards to find the end of the
string then scanning backwards to find the character.

-Slightly optimize wcswidth().

-Tell contrib srcs that we have strndup now since some of them provide
their own static definition which will blow up a buildworld.