it is more efficient than the previously used djb hash

resolve reverse dependencies after all packages have been loaded,
and avoid traversing the reverse name lists. now that we use
automatic virtual packages (soname, pkg-config, etc.) the reverse
dependency chains can become considerable longer than what it was
when the rdependency construction code was originally written.

commit 941fc1b1 uncovered a bug that directory permissions are
not updated properly at db load time if it's the default acl.

In practice this should fix to e.g. not wipe out /etc/apk/world if
final flush to /etc/apk/world.new fails.

This was prompted by an incident the other day where I ran the root
partition of an Alpine box out of space using 'apk add', and apk
helpfully wiped the contents of /etc/apk/world at the same time.

It might be tricky to try to reproduce exactly the same failure,
but from an examination of the code, setting 'rc' before the final
call to fdo_flush rather than after is one possible cause of this
behavior. (If the entire contents of /etc/apk/world.new are buffered,
and all get written out in the final fdo_flush call, and that call
fails, fdo_close will still happily rename /etc/apk/world.new to
/etc/apk/world.)

the security implications are not as high as compared to regular
tar/unzip archiver. this is because you are anyway trusting
the package to install files anywhere in the filesystem.

this serves rather as a sanity to check against errors in created
package.

the allows update-kernel script to work on tmpfs install before
cache is configured.

forgot to --amend my changes

fetch_maperror() translates error codes returned by libfetch to our error
codes. Handle those in apk_error_str(), returning error messages which
advise the user of the most likely fix.

A custom error code, EAPKSTALEINDEX, has been added for cases where
retrieving a package fails due to a HTTP error 404 or similar.

[TimoT: add also EAPKBADURL, as well as organize a bit better where the
EAPKSTALEINDEX is generated]

According to the C standards, uint32_t is defined in stdint.h.
Presumably apk is usually built against C libraries where
stdint.h is indirectly included through another header file,
but this isn't the case with the version of glibc which I am using.

there's no nice syscall to manipulate xattrs with atfd and
or open the symlink without dereferencing it (and having fd
that we can do xattrs with)

user xattrs on tmpfs are not supported no non-grsec kernels,
and many times root fs is mounted without user_xattr. Thus
to allow things to go smoothly on non-grsec kernels xattr
unsupported errors are now hidden.

xattrs can be fixed still now with "apk fix --xattrs"

use ERR_PTR mechanism, and handle it at all places.

on arm char is by default unsigned, so this caused crashes
as the ERR_PTR mechanism did not work as expected with unsigned
types. extend the array type to be signed short explicitly.

fixes #4040

issue cought by fortify

apk in alpine 3.1 already supports this optimization, but kept
the database format. now is time to start using this feature.

ref #3027

fixes #4040

fixes #4019

ref #3027

ref #3027

Package pinning was first implemented with 'p' tag. However, it
was before any release renamed to 's', and 'p' was reserved for
package provides support for which is used now.

fixes #3840

this makes 'lbu diff' and aaudit diffs nice when a world
dependency is added or removed. sorting also makes the ordering
more deterministic as the world targets constraints are always
applied in the same order. test suite updated accordingly.

using space was for backwards compatibility with apk-tools 2.2.2
and earlier (from January 2012)

the location changed in apk-tools 2.1.0 (March 2011) which was
used in Alpine Linux 2.2.