gmessages: introduce g_log_writer_default_set_debug_domains()

The problem is that resetting the environment of a proccess is not safe, this is mentioned in the man pages of the setenv, and also by the concept: when we get a constant string returned by the getenv, we need to be guaranteed that this string will exist during all that time we are using it. While setting the same env var to another value destroys this string inside of libc.

Now looking at the should_drop_message(), we can see that it just gets the environment on every call. Getting the environment is safe, but however there's no safe way to switch the logging domains selection: setenv can't be used because of the reasons listed above.

That is why g_log_set_debug_domains() is needed: it's just a safe replacement for the setenv in this case.

Now should_drop_message() still reads the environment, but does so only on the first call, after that the domains are stored in the quark, and can only be changed by g_log_set_debug_domains().

This also means that now resetting G_MESSAGES_DEBUG env var in runtime has no effect. But in any case this is not what the user should do, because resetting the environment in runtime is not correct.