rename lkerr_t to lkonf_error
fix type name in comments
more TODO
Refactor lkonf_error values.

Inspired by google's googleapis::util::Status, refactor the error codes:
* LK_INVALID_ARGUMENT replaces: LK_LKONF_NULL, LK_STATE_NULL, LK_ARG_BAD.
* LK_OUT_OF_RANGE replaces: LK_KEY_BAD, LK_VALUE_BAD.
* LK_LUA_ERROR replaces: LK_LOAD_CHUNK, LK_CALL_CHUNK.
* LK_NOT_FOUND replaces: LK_VALUE_NIL.
* LK_RESOURCE_EXHAUSTED replaces: LK_MALLOC_FAILURE.
document lkonf_error in lkonf(3)
Provide lkonf_error_to_string
prepare for getkey_double tests
need LUA_CFLAGS in AM_CPPFLAGS
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