libbb: introduce and use strftime_[YYYYMMDD]HHMMSS()

function                                             old     new   delta
strftime_fmt                                           -      53     +53
strftime_YYYYMMDDHHMMSS                                -      12     +12
strftime_HHMMSS                                        -      12     +12
human_time                                            44      43      -1
fmtstr_t                                               9       -      -9
step_time                                            361     345     -16
watch_main                                           261     232     -29
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 0/3 up/down: 77/-55)             Total: 22 bytes
   text	   data	    bss	    dec	    hex	filename
 919203	    932	  17692	 937827	  e4f63	busybox_old
 919209	    932	  17692	 937833	  e4f69	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
lineedit: implement \T \t \A \@ prompts escapes, fix \W escape, drop \!

function                                             old     new   delta
parse_and_put_prompt                                 742     793     +51
read_line_input                                     3836    3826     -10

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
lineedit: \W on "/bin" should show "bin", not "/bin"

function                                             old     new   delta
parse_and_put_prompt                                 793     785      -8

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox