merge: fix to use per-proc filenames rather than hardcoding them

merge: fix to use per-proc filenames rather than hardcoding them

The localized/utils/merge utility used hardcoded temporary filenames
to do its thing.  This prevented any Makefiles that called merge from
enabling parallel builds in order avoid the file collisions that would
result.

Now we:

- use filenames that embed the PID in them, making them unique
  per-process
- place them in /tmp, rather than the current directory

As a result, we can now re-enable parallel builds for localized
app-defaults, config, and types.