MINOR: lua: Add a flag on lua context to know the yield capability at run time

MINOR: lua: Add a flag on lua context to know the yield capability at run time

When a script is executed, a flag is used to allow it to yield. An error is
returned if a lua function yield, explicitly or not. But there is no way to
get this capability in C functions. So there is no way to choose to yield or
not depending on this capability.

To fill this gap, the flag HLUA_NOYIELD is introduced and added on the lua
context if the current script execution is not authorized to yield. Macros
to set, clear and test this flags are also added.

This feature will be usefull to fix some bugs in lua actions execution.

(cherry picked from commit a6e792ff52f895234bb1499e9df5417bf498a8f8)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>