turns out the logic does not work with overlays as expected due
to busybox symlinks being unmanaged. remove the useless option.

appears this is possible, so support it.

lookup proper directory instance as the tar output might not be
linear.

get list of overlay files from stdin, so those do not get overwritten.

less syscalls.

do not overwrite target if we have --never-overwrite and target exists.

ref #197

otherwise we always extract the file as .apk-new and the database
side just overwrites.

add cvs, svn, git and mercurial tags for creating snapshot
packages.

even more corner cases found which were broke. now all tests
should pass.

so i don't get to break version compares again.

to never ever overwrite a while in the filesystem the user knows
about. it gives the impression of extraction succeeding even though
nothing was done. this is inteded to be used only for bootstrapping
with overlay.

-t 1.3-r0 1.3.1-r0 was broke

the old code treated a symlink to directory as file; it tried
to calculate regular has of it. fix this by: 1) using no follow
on migration and pruning stats, and 2) the helper function to
check if it's point to directory and not calculate hash in that
case. fixes #188.

got broke in 0b9bfa8d which
fixed another corner case. hopefully it's good now. fixes #191.

if the dependencies are empty with only trailing new line, the
parsing was incorrect. fix the new line check.

ability embed description information to repository indexes
(e.g. repository name and version) and show it via "apk version -I".

otherwise we would not be able to pick a package that exists
as a candidate for re-installation. fixes #138.

check device major if it's tmpfs and refuse to use cache if
we've been fooled.

the first item is place holder for argv[0] = script name. it's
overwritten by the apk_pkgi_run_script.

- make virtual packages work again
- make apk del (for non-empty packages) work again

allow also overriding old (possibly bad) world dependency.
hopefully it's more bullet proof now.

accept also pkgfile in addition to pkgname. make also the signature
verification stuff work properly again with non-repository files.

this makes the database package entry smaller, and we propbably
get more fields to installed_package later too. this cleans up
the way scripts are stored and is a preparation for supporting
triggers. some parsing for trigger meta-data. ref #45.

also have the output stream support writing to temporary file
and do renameat/unlinkat on close depending on if all writes
succeeded or not.

and take checksums for symlinks too.

otherwise, non repository files present in cache are not
considered for installation (e.g. when booting from cdrom).