BUG/MINOR: hlua_fcn: fix Patref:set() force parameter

BUG/MINOR: hlua_fcn: fix Patref:set() force parameter

Patref:set(key, val[, force]) takes optional "force" parameter (defaults
to false) to force the entry to be created if it doesn't already exist

To retrieve the value, lua_tointeger() was used in place of
lua_toboolean(), and because of that force is not enabled if "true"
is passed as parameter (only numbers were recognized) despite the
documentation mentioning that "force" is a boolean.

To fix the issue, we replace lua_tointeger by lua_toboolean.

Also, the doc was updated to rename "bool" to "boolean" for the "force"
parameter to stay consistent with historical naming in the file.

No backport needed unless 9ee37de5c ("MINOR: hlua_fcn: add Patref:set()")
is.