Enhance the output sync mode.

Create a new file, output.c, and collect functions that generate output there.
We introduce a new global context specifying where output should go (to stdout
or to a sync file), and the lowest level output generator chooses where to
write output based on that context.

This allows us to set the context globally, and all operations that write
output (including functions like $(info ...) etc.) will use it.

Removed the "--trace=dir" capability.  It was too confusing.  If you have
directory tracking enabled then output sync will print the enter/leave message
for each synchronized block.  If you don't want that, disable directory
tracking.
Support the -Orecurse option properly.

In this mode we still collect all the output from a given target and
dump it at once.  However we don't treat recursive lines any differently
from non-recursive lines.  Also we don't print enter/leave messages
after every dump.  However we do ensure that we always print them once
to stdout, so the parent make will collect it properly.