dovecot: add new option to install pigeonhole.
dovecot: add new option to install pigeonhole.

Based on the code by @wahjava from
https://github.com/Homebrew/homebrew/pull/44319.

Closes #44319.
Closes #46331.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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