http://ewontfix.com/11/

remount to read-write before trying to create the cache directory
subdirs. fix a fd leak that might prevent remounting back to rw.

If GNU wget is available, then we want use it as it gives the user
possibility to set misc options, such as bandwidth limit in wgetrc.

It might also be that busybox is not available in case bootstrapping
alpine from other distros (think debian vserver host creating an
alpine guest)

In any case we, fall back to busybox wget in case wget libs are getting
upgraded. (see http://redmine.alpinelinux.org/issues/347)

I think there used to be problems with using http_proxy on bb wget,
but those have been fixed for quite some time. This fixes #347.
We should probably use libcurl or similar library eventually, but
I'm not entirely sure if/when we want that.

 * prunes the child pid to avoid zombies
 * handles the errors so e.g. file-not-found is reported properly

this way we never change cwd, and relative filenames are always
parsed consistently. this also helps filename construction in many
places. this patch also changes '--root' to override location of
all configuration to be in the new root. previously it depended
on the file which one was used.

Cache non-local index files always locally.
Introduce 'update' applet to force refresh of cached index files.
Fixes #19.