dpkg: code shrink

dpkg: code shrink
dpkg: add documentation from bug 3644

function                                             old     new   delta
remove_package                                       318     333     +15
unpack_package                                       541     552     +11
purge_package                                        242     247      +5
all_control_list                                      55      53      -2
all_control_files                                     44      40      -4
add_split_dependencies                               709     703      -6
search_status_hashtable                              148     133     -15
search_package_hashtable                             137     122     -15
search_name_hashtable                                134     118     -16
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/6 up/down: 31/-58)            Total: -27 bytes
   text    data     bss     dec     hex filename
 808093     611    6924  815628   c720c busybox_old
 807972     611    6924  815507   c7193 busybox_unstripped