Hide various mbstate implementation details.

...by inlining them.

Also fix a couple of harmless bugs in passing. I've added tests, but in
both cases I don't think it was actually possible to hit the bad behavior:
we'd hit another test and fail immediately after in an externally
indistinguishable way.

Bug: N/A
Test: readelf
Change-Id: I8466050b0bfe2b7b94c76b383cf10c1d9d28debd
diff --git a/libc/bionic/mbrtoc16.cpp b/libc/bionic/mbrtoc16.cpp
index 6878a11..2180516 100644
--- a/libc/bionic/mbrtoc16.cpp
+++ b/libc/bionic/mbrtoc16.cpp
@@ -55,7 +55,7 @@
   char16_t trail = mbstate_get_byte(state, 1) << 8 |
                    mbstate_get_byte(state, 0);
   *pc16 = trail;
-  return reset_and_return(mbstate_get_byte(state, 3), state);
+  return mbstate_reset_and_return(mbstate_get_byte(state, 3), state);
 }
 
 size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps) {
@@ -76,13 +76,13 @@
   if (__MB_IS_ERR(nconv)) {
     return nconv;
   } else if (nconv == 0) {
-    return reset_and_return(nconv, state);
+    return mbstate_reset_and_return(nconv, state);
   } else if (c32 > 0x10ffff) {
     // Input cannot be encoded as UTF-16.
-    return reset_and_return_illegal(EILSEQ, state);
+    return mbstate_reset_and_return_illegal(EILSEQ, state);
   } else if (c32 < 0x10000) {
     *pc16 = static_cast<char16_t>(c32);
-    return reset_and_return(nconv, state);
+    return mbstate_reset_and_return(nconv, state);
   } else {
     return begin_surrogate(c32, pc16, nconv, state);
   }