manager: configurable Meson directory

manager: configurable Meson directory

This commit makes it more convenient to change the configuration of the
build directory of `kresd` when using Knot Resolver Manager. It adds a
new `./poe configure` command, which optionally takes the same arguments
as the standard `meson configure` command.

The `./poe run` command now requires running `./poe configure` at least
once to set up the build directory. If the directory has been configured
before this commit (i.e. `./poe run` has been executed at least once),
no extra action is required, as the directory structure remains the
same.

The commit also removes the `manager` configuration option from Meson as
we were not using it and it was broken and potentially confusing to
newcomers.