cli: set up shared command-line arguments for indexing

cli: set up shared command-line arguments for indexing

We have an indexopts structure for manipulating indexing in different
ways, but we also have three command-line invocations that can trigger
indexing: new, insert, and reindex.

This changeset prepares a common parser that these subcommands can
share.

Note: if the deprecated crypto.gpg_path configuration option is set to
anything other than "gpg", we ignore it (and print a warning on
stderr, if built against gmime < 3.0).

At the moment, it's just --try-decrypt, but others will likely follow.