support PIM parameter for VeraCrypt compatible devices

Closed username-removed-1089654 requested to merge nachtgeist/cryptsetup:master into master

This patch adds the --veracrypt-pim=INT command-line parameter to support specification of a Personal Iteration Multiplier, which affects the number of iterations for key derivation from the entered password.

Also the main manpage is updated.

Closes #307 (closed)