Move extraction of encryption and rotation options to separate functions

[felixfontein]

Preparation for a rewrite of #1343, which aims at implementing #1333 (Make encrypt, decrypt, rotate, set, and edit proper commands).

Currently contains a rebased version of #974, since that adds more encryption options.

This simplifies the main command handler to a more manageable ~180 lines of code, and heavily reduces the copy'n'paste that would happen in an implementation like #1343.

[felixfontein]

An unintended side-effect: if before, --encrypt --set has been specified together, the --encrypt fails (since the file is already encrypted), but the error returned by it is ignored by the old code and overwritten by --set's error (which is nil). 4e8796e restores the old behavior. (I'll create a PR to issue a warning if such commands are combined.)

[felixfontein]

I modified that commit to 05ff087: first, --decrypt and --rotate have the same effect as --set on --encrypt (see #1388 which adds a warning of more than one command is used). Also the change for --rotate was already introduced in #1317 (and is already released), so I don't want to undo that behavior change. (It was fixing something even more broken.)

[felixfontein]

#1389 is this PR without #974, so it can be reviewed and merged sooner.

[felixfontein]

I moved the main commit to #1392, rebased upon #1389. I'm closing this one in favor of #1392 (and of course #1389).