touch: implement -d --date (our own testsuite needs that)

touch: implement -d --date (our own testsuite needs that)

function                                             old     new   delta
static.uname_longopts                                  -     137    +137
touch_main                                           231     360    +129
static.touch_longopts                                  -      32     +32
validate_tm_time                                       -      28     +28
packed_usage                                       26616   26624      +8
date_main                                            687     686      -1
static.longopts                                      162       -    -162
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 2/1 up/down: 334/-163)          Total: 171 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>