Adds survivor machete.

As suggested by @John-Candlebury here:
http://smf.cataclysmdda.com/index.php?topic=8444.msg194374#msg194374.

An improved version of the machete, the survivor machete is an
autolearned craftable improvement that adds a couple points of damage
and a few techniques to the weapon.

Can occasionally spawn in survivor zed loot.
Derp.
Grammar.
Secondary skill.
Update armor.json

Adds dusters. They're modeled after the trenchcoat items but also cover the legs. Their volume, weight, and components needed to craft are 35% higher than trenchcoats.
Update recipe_armor.json

The duster recipes. Have a 35% higher component requirement than trenchcoats.
Update item_groups.json

Simply adds duster spawn chances to the same places trenchcoats spawn, and at the same spawn rates.
Fix TypeError from silly use of None defaults

Repeat after me: None is not iterable. I shall not attempt to iterate
None. I shall not pass None as the default value for a missing
dictionary key unless I want a TypeError.
Don't leave files open. It's what `with` is for.
Small mapgen fix
Added an option in the martial arts menu to prevent accidental wielding of items when fighting unarmed. (keep hands free)
Remove unused function itype::is_covering

This is handled by the item class directly (items can have differently sided coverage, differing from the coverage of the it_armor type).
minor fix to fix in filename logic
shorten filename length, was not included in prior fix
Initial survivor's arsenal update
Json eror
More json fixes
Even more json fixes. Going to fix my compiler after these
Remove the prompt for foraging

I can't think of a reason why you'd particularly not want to forage, and it makes wilderness scavenging much less tedious.
Remove unused parameter

And if I'm reading things correctly, interrupting the activity should be handled in the activity handling code and not here, correct?