No way to change pinentry program

[wlcx]

Currently where there is seemingly no way to configure yubikey-agent to use a pinentry program other than /usr/bin/pinentry. Of course, update-alternatives or another symlink approach can be used where available to point this file elsewhere, but if the user doesn't have root or is unwilling to futz with "system level" stuff like this, they are out of luck.

This is because gopasspw/pinentry calls and parses the output of gpgconf to determine the path to the pinentry binary. From reading the source of gpgconf, /usr/bin/pinentry is hard-coded - there's no way seemingly to change it with a config file.

In contrast, twpayne/go-pinentry determines the pinentry binary to use by parsing gpg-agent.conf, which seems a much more logical and useful approach. As such #113 would fix this!