Kernel+LibC: Tidy up sys$ttyname() and sys$ptsname()

- Remove goofy _r suffix from syscall names.
- Don't take a signed buffer size.
- Use Userspace<T>.
- Make TTY::tty_name() return a String instead of a StringView.

Kernel/TTY/SlavePTY.cpp

@@ -54,7 +54,7 @@ SlavePTY::~SlavePTY()
     DevPtsFS::unregister_slave_pty(*this);
 }
 
-StringView SlavePTY::tty_name() const
+String SlavePTY::tty_name() const
 {
     return m_tty_name;
 }