tty: Rename helpers to reflect the type of peers
tty: Rename helpers to reflect the type of peers

 - parse_index -> parse_pty_index since it's PTY specific
 - pty_is_master -> tty_is_master because it will test
   not only PTY peers but other types as well
 - pty_is_hung -> tty_is_hung for same reason

Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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