pull: add --direct mode + make various eatures optional + explicit checksum verification

pull: add --direct mode + make various eatures optional + explicit checksum verification

This does what the previous commit did for systemd-import the same way
for systemd-pull.

It also adds one more thing: the checksum validation is extended, in
addition of doing SHA256SUMS/gpg verification it is now possible to
immediately specify a hash value on the command line that the download
needs to match. This is particularly useful in --direct mode as we can
download/decompress/unpack arbitrary files and check the hash of the
downloaded file on-the-fly.