[Fix] Handle native boolean values in lua_util.toboolean()

[Fix] Handle native boolean values in lua_util.toboolean()

toboolean() only handled string and number inputs, falling through to
the error branch for native Lua booleans and always returning false.
This broke the fuzzy_check "checks" configuration block since UCL
converts boolean values to native Lua booleans, and apply_checks_overrides
in lua_fuzzy.lua passes them through toboolean() via bool_opt().

Fixes: #5775