date: factor out date parsing (in preparation for touch -d)

date: factor out date parsing (in preparation for touch -d)

function                                             old     new   delta
parse_datestr                                          -     391    +391
sha512_process_block128                             1283    1310     +27
buffer_fill_and_print                                179     196     +17
nexpr                                                826     840     +14
unzip_main                                          1931    1939      +8
popstring                                            134     140      +6
qrealloc                                              33      36      +3
builtin_umask                                        121     123      +2
evalvar                                             1365    1363      -2
changepath                                           194     192      -2
do_compress                                         1698    1688     -10
hwclock_main                                         340     329     -11
cmdputs                                              414     402     -12
identify                                            4343    4329     -14
date_main                                           1186     687    -499
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 7/7 up/down: 468/-550)          Total: -82 bytes

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