adduser: Add -k /path/to/skel

adduser: Add -k /path/to/skel

function                                             old     new   delta
.rodata                                           157069  157133     +64
adduser_main                                         921     936     +15
adduser_longopts                                     103     110      +7
packed_usage                                       30230   30236      +6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/0 up/down: 92/0)               Total: 92 bytes

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>