dpkg_deb: slight code shrink

dpkg_deb: slight code shrink
ar: reuse existing ar unpacking code
get_header_tar: handle autodetection for tiny .tar.gz files too
unarchive.h: do not include CONFIGed out things

function                                             old     new   delta
get_header_tar                                      1521    1534     +13
dpkg_deb_main                                        400     380     -20
ar_main                                              260     196     -64
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(add/remove: 0/0 grow/shrink: 1/2 up/down: 13/-84)            Total: -71 bytes