nsenter: new applet

nsenter: new applet

function                                             old     new   delta
nsenter_main                                           -     663    +663
.rodata                                           155147  155612    +465
packed_usage                                       30536   30708    +172
nsenter_longopts                                       -     116    +116
open_by_path_or_target                                 -      58     +58
applet_names                                        2518    2526      +8
applet_main                                         2920    2928      +8
------------------------------------------------------------------------------
(add/remove: 4/0 grow/shrink: 4/0 up/down: 1490/0)           Total: 1490 bytes
   text	   data	    bss	    dec	    hex	filename
 827956	   4078	   9080	 841114	  cd59a	busybox_old
 829214	   4086	   9080	 842380	  cda8c	busybox_unstripped

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>