Look up top-level config options by enum (= integer), not by name
string.

The new struct ConfigOption is the compile-time description, and the
global array "config_params" contains the actual values.

Let STL manage the WatchDirectory allocations.

Simplify error handling.

Fixes crash when another object attempts to access the EventLoop
during destruction.

Eliminate the C wrappers.

BufferedSocket has just an input buffer, and FullyBufferedSocket adds
the output buffer.

Don't use va_list.

Add the interface ArchiveVisitor.