Fix <wctype.h>.

* Fix the return type of towlower_l/towupper_l.
* Implement wctrans/wctrans_l/towctrans/towctrans_l.
* Move declarations that POSIX says are available from both <wchar.h> and
  <wctype.h> to <bits/wctype.h> and include from both POSIX headers.
* Write the missing tests.

Change-Id: I3221da5f3d7e8a2fb0a7619dc724de45f7b55398
diff --git a/libc/libc.arm.map b/libc/libc.arm.map
index 4e69bf1..8628ed1 100644
--- a/libc/libc.arm.map
+++ b/libc/libc.arm.map
@@ -1291,6 +1291,10 @@
     sigrelse;
     sigset;
     sync_file_range;
+    towctrans;
+    towctrans_l;
+    wctrans;
+    wctrans_l;
 } LIBC_N;
 
 LIBC_PRIVATE {