Replace default files location about maddy HOT 3 CLOSED

I think we should define devdirs build tag to change default directories to make it easier to develop maddy (I really don't like the idea of having testing DB in /var/lib/maddy).
Need to think about a better name for such tag though, usecwd?.

Aux. binaries search path (auth helpers for now): $MADDYBIN + $GOBIN + $PATH, $MADDYBIN defaults to /usr/libexec/maddy or empty (which means it will use current directory) if devdirs is on.
State directory: $MADDYHOME, defaults to /var/lib/maddy or empty (current directory) if devdirs is on. Used for database of default storage backend and queues.
Configuration: /etc/maddy/Maddyfile is the main file, imports should be resolved relative to /etc/maddy, i.e. import foo will read /etc/maddy/foo.
Log files (for #26, if the file path is not absolute): $MADDYLOG, defaults to /var/log/maddy or current directory if devdirs is on.

Non-devdirs values (/usr/libexec/maddy/, /var/lib/maddy, /etc/maddy) are also settable using -X linker flag so they can be customized by packagers without the need to patch sources.

@emersion, what do you think?

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I'm not a big fan of devdirs.

What about defaulting to /etc/maddy and providing a flag to override the config file path?

Everything (logfiles path, DB path) should be configurable in the config file anyway. So you could have a dev config file.

Non-devdirs values (/usr/libexec/maddy/, /var/lib/maddy, /etc/maddy) are also settable using -X linker flag so they can be customized by packagers without the need to patch sources.

+1, this will be required for custom prefixes

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Alright, that makes more sense.

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