Move a misplaced comma.

I had misplaced a comma in `modify_unit_attacks.lua`, resulting in
attempts to run that script erroring out with syntax errors.

This commit fixes the issue by moving the comma to its proper place.
`modify_unit_attacks`: Split into more functions.

Refactor `modify_unit_attacks.lua`, splitting most of the inner loop
into two local functions, `computeDiff` and `computeAttrDiff`.

This results in the script no longer having lines over 80 columns long
when viewed with 8-space-wide horizontal tabs, rather than the
4-space-wide horizontal tabs with which I originally wrote it.

This should also result in the affected loop body being more easily
comprehensible.
modify_unit_attacks: Use `lp8.eval`’s `run_err`.

Alter `modify_unit_attacks` to use `string/eval`’s new(ish) `run_err`
parameter, which was introduced in commit
a3e582c25be6dc0750b21c2513f5c0c9f8ff257a.
modify_unit_attacks: Rename some variables.

In `modify_unit_attacks.lua`, rename the `atk` variables to `attack`,
and rename the `atkcfg` variable to `diff`.
Add Makefile.

Add a Makefile with `test` and `release` targets, as a convenience
wrapper for the test and release scripts.
release.zsh: Run WAM under Python 2.

In `scripts/release.zsh`, explicitly run the `wesnoth_addon_manager`
script under Python 2.7 or Python 2.6, rather than the system’s default
Python version.

The script will abort with an error message if neither Python 2.7 nor
Python 2.6 appears to be available.
release.zsh: Use `exit` rather than `return`.

In `scripts/release.zsh`, use the `exit` built-in command rather than
the `return` built-in command to exit the script with a specified exit
status code.
string/eval: Switch error handler parameter order.

In `string/eval`, switch the orders of the parameters of the error
handler callbacks, for better interoperability with other interfaces.