Defines starting with `n' in kern

• NA kern/tty.c:89
• nch_back kern/vfs_cache.c:72
• nch_forw kern/vfs_cache.c:71
• NCL_INIT kern/uipc_mbuf.c:57
• NL kern/tty.c:92
• NO kern/tty.c:88
• NPTY kern/tty_pty.c:58
• NTBS kern/tty_tb.c:97