calc 2.16.1.0 release

The following are the changes in this release:

Added a new `.clang-format` file as the basis for using the
`clang-format(1)` tool to format calc source in a consistent way.
This code re-formatting will make it easier to work in calc v3 code
changes in the future by making the calc source much more consistent.

Removed `.clang-format` exclusion from `.gitignore`.

Modified `func.c` to prevent the `clang-format(1)` tool from formatting
the `builtins[]` table that builtin make rule in `help/Makefile` uses to
format the `help/builtin` file.  We can live with the `builtins[]`
table as it is today, for now.

Add `make clang-format` make rule to top level `Makefile` and
to the `custom/Makefile`.

Reformatted C code as per the `clang-format(1)` and the
`.clang-format` file: using the `make clang-format` make rule.