Defines starting with `n' in kern

• NA kern/tty.c:94
• nch_back kern/vfs_cache.c:71
• nch_forw kern/vfs_cache.c:70
• NCL_INIT kern/uipc_mbuf.c:63
• NDDB kern/ddb.h:1
• NL kern/tty.c:97
• NO kern/tty.c:93
• NPTY kern/tty_pty.c:72
• NTBS kern/tty_tb.c:96