strerror: allow user hook to comply with POSIX rules

* libc/string/strerror.c (strerror): Split body into...
(_strerror_r): ...new reentrant function.
* libc/string/u_strerr.c (_user_strerror): Update signature.
* libc/include/stdio.h (_strerror_r): New prototype.
* libc/posix/collate.c (__collate_err): Adjust callers.
* libc/stdio/perror.c (_perror_r): Likewise.
* libc/string/strerror_r.c (strerror_r): Likewise.
* libc/string/xpg_strerror_r.c (__xpg_strerror_r): Likewise.

newlib/libc/string/u_strerr.c

@@ -1,8 +1,10 @@
 #include <_ansi.h>
 
 char *
-_DEFUN(_user_strerror, (errnum),
-       int errnum)
+_DEFUN(_user_strerror, (errnum, internal, errptr),
+       int errnum _AND
+       int internal _AND
+       int *errptr)
 {
   return 0;
 }