Allow configurable pinentry options

On Debian, the Keybase Go client uses the GTK pinentry for typing the Keybase passphrase.My Keybase passphrase is long, randomly-generated, and impossible to type by hand. Since pinentry prevents pasting, this makes authenticated operations inconvenient. I would like to be able to edit my config file to:
1. select the pinentry to use, and
2. configure the frequency of passphrase prompts.

For instance, I'd prefer to disable passphrase prompts entirely, and replace the ncurses path with a shell script which decrypts the passphrase using a smartcard. Others may prefer a passphrase prompt for every operation, or to use the ncurses pinentry rather than the GTK one.

The issue names no files, tests, or entry points. Start by locating the Keybase Go client's configuration handling and current Debian pinentry paths; done means configuration can select the pinentry and control prompt frequency, including the requested shell-script flow.

Written by the indexing model from the issue text.