Explicitly iterating the context is more flexible, as we can control the
parameters how long we iterate. GMainLoop is essentially a (thread-safe)
iteration around one boolean flag (controlled by g_main_loop_run() and
g_main_loop_quit()). We can maintain that boolean flag ourselves.
Originally, we would define G_LOG_DOMAIN via CFLAGS arguments.
Since commit 341b6e0704 ('all: change G_LOG_DOMAIN to "nm"') we would
instead set it in source and uniformly define it as "nm".
The reasons are that most parts of our source should not use g_log() directly,
and there is an aim to avoid special CFLAGS to simplify the build setup.
However, dispatcher indeed uses g_log() for logging, so the value there
is important.
Fix that, but this time by setting the define in source not via
CFLAGS.
Fixes: 341b6e0704 ('all: change G_LOG_DOMAIN to "nm"')
