Preserve the configured logging verbosity unless the user actually
changes it, rather than capping it to LINFO on release builds.

Rename LogManager::m_level to m_effective_level and distinguish between
the config and effective level in various function/variable names.

Make m_effective_level atomic to prevent data races when setting the
effective log level from the config changed callback.

This fixes a memory leak that would occur when the Android frontend
calls LogManager::Init more than once in order to reload settings.

Note that the log window listener is now owned by LogManager instead of
by the frontend, making it consistent with the other log listeners.

Storing the log type names in a map results in them getting re-sorted by
their keys, which doesn't quite give us the sorting we want. In
particular, the Achievements category ended up being sorted at R (for
RetroAchivements) instead of at A. Every use of the map is just
iterating through it, so there's no real reason why it has to be a map
anyway.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Provides a basic extension to the interface to begin migration off of
the printf-based logging system.

Everything will go through macros with the same style naming as the old
logging system, except the macros will have the _FMT suffix, while the
migration is in process.

This allows for peacemeal migration over time instead of pulling
everything out and replacing it all in a single pull request, which
makes for much easier reviewing.