MINOR: hlua: rename "tune.lua.preserve-smp-bool" to "tune.lua.bool-sample-conversion"

MINOR: hlua: rename "tune.lua.preserve-smp-bool" to "tune.lua.bool-sample-conversion"

A better name was found for the option implemented in ec74438
("MINOR: hlua: add option to preserve bool type from smp to lua")

Indeed, "tune.lua.preserve-smp-bool {on | off}" wasn't explicit enough
nor did it encourage the adoption of the new "fixed" behavior (vs
historical behavior which is now considered as a bug).

Thus it becomes "tune.lua.bool-sample-conversion { normal | pre-3.1-bug }"
which actively encourage users to switch the new behavior after having
patched in-use Lua script if needed. From a technical point of view,
the logic remains the same, as the option currently defaults to
"pre-3.1-bug" to prevent script breakage, and a warning is emitted if
the option isn't set explicily and Lua is used.

Documentation and regtests were updated.

Must be backported in 3.1 with ec74438 and f2838f5 ("REGTESTS: fix
lua-based regtests using tune.lua.smp-preserve-bool")