apparently it needs to have both PROT_READ and PROT_WRITE. and
it needs to be MAP_SHARED for the writing to be effective. oh,
and the data needs to be preallocated with ftruncate; otherwise,
one gets SIGBUS.
change the index generation to do old index, or the new style index
where package identity is sha1 of control block and it's contained
within an .tar.gz to allow signing in future.
If /etc/apk/cache is a symlink to directory, a copy of all installed
packages is stored there, and the index of remote repositories will
be there instead of /var/lib/apk. This enables to reconstruct running
system during boot.
Left as todo: remove cached copy when the package is removed, and
additional apk applet to download missing packages to cache and/or
remove extra items.
Checksum of installed is computed on the fly when extracting them
and it'll be saved to fdb. When installing config files those are
diverted with suffix .apk-new if earlier version of same file with
local changes exist.