Get maintainer from a field instead of from a comment

Currently, abuild gets the maintainer of a given APKBUILD from the # Maintainer comment. This is unintuitive as in most cases, comments are ignored by anything but the human reading the code. As such, people often don't realise that they need to bump the pkgrel when changing the maintainer line.

To resolve this, I think the maintainer comment should be changed to a maintainer field, as in something like this:

maintainer="Stefan Hansson <newbyte@postmarketos.org>"

This would make it clearer, I think, that the maintainer information actually gets embedded in the resulting package.

As a migration path to make the transition easier, abuild could understand both but print a warning when a maintainer comment is found.