build: Introduce cmdlist_dir.

build: Introduce cmdlist_dir.

This puts all files generated by command_util.bash into a new
subdirectory inside the build directory. This way we don't need
to special-case these files any more in .gitignore, Doxyfile, make
clean. The compiler and depend.sh need to know this directory, so
we add -I$(cmdlist_dir) to CPPFLAGS and introduce a new parameter
for depend.sh.

To prevent cpp from including stale versions of the *command_list.h
files in the top level directory (left over from a previous build)
this commit renames the generated files slightly. For example,
audiod_completion.h becomes audiod.completion.h.