Fix linkage of grantpt(3).

Also clean up the implementation of all the pty functions, add tests,
and fix the stub implementations of ttyname(3) and ttyname_r(3).

Bug: https://code.google.com/p/android/issues/detail?id=58888

(cherry picked from commit 4916706cfe)

Change-Id: I5cb7a1c17b156456e4c4818e65f256eb8d045424

libc/include/unistd.h

@@ -170,7 +170,7 @@ extern char *optarg;
 extern int optind, opterr, optopt;
 
 extern int isatty(int);
-extern char* ttyname(int);
+extern char* ttyname(int) __warnattr("ttyname is not thread-safe; use ttyname_r instead");
 extern int ttyname_r(int, char*, size_t);
 
 extern int  acct(const char*  filepath);