m_config: remove an old temporary hack

Actually rewrite most of the option management code. This affects how
options are allocated, and how thread-safe access to them is done.

One thing that is nicer is that creating m_config_cache does not need to
ridiculously recreate and store the entire option list again. Instead,
option metadata and option storage are now separated. m_config contains
the metadata, and m_config_data all or parts of the actual option
values. (m_config_cache simply uses the metadata part of m_config, which
is immutable after creation.)

The mentioned hack was introduced in commit 1a2319f, and is the
global state around g_group_mutex. Although it was "benign" global
state, it's good that it's finally removed.