Add support for contents checksums. This is meant to be used for tarballs
like the ones coming from https://*.googlesource.com/ which on every download
change their outer checksum due to being packaged with different date/time
stamps or similar.
If such a distfile still contains the same files as the author of a template
found, then the checksum of its contents can be specified by prepending a
commercial at (@) to the corresponding checksum="..." line.
Depending on the format of the distfile its entire contents will then be
piped into sha256sum and the resulting digest is verified against what
follows after the @.
If a distfile changed its checksum (SHA256) without at the same time
also changing its version, the distfile cache needs to be purged.
This is done by verifying only for already existing distfiles that the
specified checksum matches. If it does not, the file and its entry in
/host/sourcers/by_sha256 are purged using the inode number of the file.
The second loop then as usual fetches the distfiles and verifies their
checksum against the list entry.
+ drbd-utils fix checksum and also build + cross build
The idea is to remove some burden from the original download
locations for distfiles by specifying one or more mirror locations.
+ Works with http:// or ftp:// mirrors for all $XBPS_CHROOT_CMD variants
+ Works with file:// mirrors together with uchroot and proot
