Pointed-out-by a warning.

"signed and unsigned type in conditional expression"

It's ugly, but it will do.

This reverts commit 0f245d7b.

Conflicts:

	src/misc/update.c

This reverts commit ebfd9455.

This reverts commit 97f83156.

Move cryptography specific functions into their own file, together with
functions downloading public key / signatures
Factorize common code for generating signatures (hash_sha1_from_*)
Remove unused update_WaitDownload (was OSX specific)