templater: load aliases from [templatealias] section in map file This seems sometimes useful as an alias can be a function, but a template fragment can't.

templater: load template fragments from [templates] section in map file This allows us to %include map-cmdline.<style> file in our .hgrc files. The syntax is slightly different as hgrc doesn't support loading an external template file, but map-cmdline files don't use this feature, so the syntax can be considered identical in practice. Unnamed section is remapped for backward compatibility.

config: allow remapping the default section The next patch depends on it. It doesn't make sense that the default section can't be remapped with {'': whatever}.