find_directory(): Add architecture support

* Move PathBuffer helper class out of find_paths.cpp into its own
header.
* find_directory():
- Make use of MemoryDeleter to simplify things.
- Make use of PathBuffer for a simpler and more correct handling.
- Make B_UTILITIES_DIRECTORY to B_APPS_DIRECTORY. /boot/utilities
doesn't exist anyway.
- Resolve the concerned constants to the architecture specific
subdirectory, when called in a secondary architecture context, just
like find_path*().

find_directory(): Add architecture support

* Move PathBuffer helper class out of find_paths.cpp into its own
header.
* find_directory():
- Make use of MemoryDeleter to simplify things.
- Make use of PathBuffer for a simpler and more correct handling.
- Make B_UTILITIES_DIRECTORY to B_APPS_DIRECTORY. /boot/utilities
doesn't exist anyway.
- Resolve the concerned constants to the architecture specific
subdirectory, when called in a secondary architecture context, just
like find_path*().