This changes the LOG macro to be varadic removing the need for all
callsites to have double bracketing and allows for future improvement
on how we use the logging macros.
The callsites were changed with coccinelle and the changes checked by
hand. Compile tested for several frontends but not all.
A formatting annotation has also been added which allows the compiler
to check the parameters and types passed to the logging.
Despite the fact AmigaOS filesystems run as separate processes,
DOS filesystem writes occur synchronously waiting for the disk
write to finish before returning. Here we have effectively taken
the core's fs backing store and extended it slightly so the store
operation is spun off into a new process, running at a lower task
priority than NetSurf itself. This stops the UI pauses that occur
shortly after loading pages as NetSurf commits the cache to disk.
Somewhat experimental, pending more extensive user testing.
