Revert "main: remove clearsign option"

This reverts commit 83e0e3ad5818c125d1b1936619576d0c032d509c.

The logic behind removing clearsign was somehow flawed by momentary
indisposition.
actions: added clearsign to parameters
bvector: string/bvector conversion utility
actions: encryption
actions: decrypt
actions: don't leak sencode on parsing failure