remove duplicate const qualifier
adjust for lua 5.2 differences in syntax errors
rearrange lua 5.1 vs 5.2 compat testing

Use a more understandable idiom for testing for lua >= 5.2
exercise getkey_double
regen for getkey_double
reorder exercise_get_boolean args

Change the order of exercise_get_boolean() args so that it's
easier to copy&paste lines to other type tests.
reorder exercise_get_double args

Change the order of exercise_get_double() args so that it's
easier to copy&paste lines to other type tests.
convert exercise_get_integer to new sig

Convert exercise_get_integer to new function signature.
Use NULL instead of 0 for empty lkonf_keys array.
sync get_integer tests with get_double
exercise getkey_integer
regen
convert exercise_get_string to new signature
test getkey_string
regen
update TODO

DONE: unittest_lkonf for getkey_*
TODO: support lua 5.2 ?