These names are reserved for the compiler/library implementations.

Remove DTIME macro and its uses, too

- Add warning log level
- Change debug_log_level setting to enumeration string
- Map Irrlicht log events to MT log events
- Encapsulate log_* functions and global variables into a class, Logger
- Unify dstream with standard logging mechanism
- Unify core.debug() with standard core.log() script API

Change a remaining assert(0) call to FATAL_ERROR(msg).
There was a regression since commit

ced6d202 "For usages of assert() that are meant to persist in Release builds (when NDEBUG is defined), replace those usages with persistent alternatives"

where when an "uncatched" exception is thrown inside a "side thread",
the program doesn't abort anymore.

This led to the problem @netinetwalker experienced where the emergethread
got an unhandled exception for loading a mapblock while redis was loading,
(see #3196) and then jmped outside its loop to work down its queue.
This resulted in the server not doing any emerges anymore.

For usages of assert() that are meant to persist in Release builds (when NDEBUG is defined), replace those usages with persistent alternatives

Remove software exception translator function, simplifying exception handler
macros.  FatalSystemExceptions are left unhandled.

Two new configuration options are added:
     - "enable_ipv6" to enable/disable the overall use of IPv6
     - "ipv6_server" to enable/disable the use of IPv6 sockets when running
       a server (when "enable_ipv6" is enabled)

Switch the license to be LGPLv2/later, with small parts still remaining as GPLv2/later, by agreement of major contributors

Use the logger; also, default to not showing much crap in console. Use --info-on-stderr to enable crap.