Implement <pty.h>.

Based on the package/apps/Terminal implementation. I'll switch them over
shortly. This also lets us build the toybox version of netcat.

Change-Id: Ia922a100141a67409264b43b937eeca07b21f344

libc/include/utmp.h

@@ -91,6 +91,8 @@ int utmpname(const char*);
 void setutent();
 struct utmp* getutent();
 
+int login_tty(int);
+
 __END_DECLS
 
 #endif /* _UTMP_H_ */