fix programming microcontroller through form

The handling of the "quit" pseudo-field meant that the microcontroller
couldn't be programmed with explicit code, only with the examples.
The "quit" can actually be ignored: what matters for programming the
controller is whether the "code" field was supplied.
Fix #140 once again
Don't allow non-inventory items as ingredients

Some mesecon wires (the turned-on nodes) that were
not_in_creative_inventory and should never appear in an actual inventory
were also mesecon_conductor_craftable.  This is liable to make a craft
guide show them as potential ingredients, due to the use of the group
in recipes.
Make sure #160 cannot be exploited to make servers crash.

This is not exactly a fix for the issue, because extremely
large circuits (3000+ conductors) still won't work with this applied.
This simply aborts any execution if there is the danger of a stack overflow.
Fix the commandblock's check for quiting the formspec

The submit button also sends a quit field.
Add node detector, which works like the player detector but detects a specific nodename (or any node except air) in front of it.
Fix #164
Merge branch 'master' of github.com:Novatux/minetest-mod-mesecons
Localize a few variables, add "GET" command to node detector.
Merge branch 'master' of github.com:Novatux/minetest-mod-mesecons
Merge pull request #168 from ShadowNinja/fix-commandblock-quit

Fix the commandblock's check for quiting the formspec