Fix .db.sig skip check- don't try to download at all

[srcreigh]

RE #128 #126

This change skips downloading all *.db.sig files.

Rationale to skip downloading :

• Due to how pacoloco treats 404 responses as an error, it needs to send a request to every mirror to give a 404 for a .db.sig request.
• This is slow, in particular it introduces a 10s lag to system upgrade if an old mirror on the list hangs.
• Consistency with pacman: even with a mirror that would hang, pacman doesn't hang for 10s to download sig files.

This change also fixes the log spam about .db.sig files. The prior check added in PR #120 doesn't seem to work since u is the upstream URL.