Mention that PUBKEY is a path not a string.
Refuse to create repo if it cannot be checked.
Fix bug with signature checking.

Actually, due to this bug such repos were not functional at all.

Issue:	#813
Fix repository update if it was not modified.
Do not retry to fetch non-existent files.

Miracles won't happen: if a server tells that a file does not exist,
then we shouldn't try to fetch it again and again.
Respect hardlinks during flatsize calculation.

Issue:	#812
Reported by:	@gelraen
Fix is_hardlink definition.

Reported by:	@gelraen
Fix string comparision for reponame.
Store manifest digest for local packages.
Detect equal remote and local packages by digest.
Fix issue with upgrades finding.
Improve output if no packages are going to be installed.
Do not solve jobs with no requests.
Rename `is_hardlink` to `check_for_hardlink`.

Suggested by:	@gelraen
Update digests in the local database.

Warning: can be very slow on large databases.
Search for upgrade candidates first prior to upgrade.
Try to use package from cache for manifestdigest.
Do not fail on empty checksum.
Initial checksums API.
Add braces around long block