Resolve #361 by zapping existing chroots after init (#385)

This extends zap() to add a 'no_confirm' option (False by default), and
zap() is now called by init with no_confirm=True to automatically zap
any existing chroots after the user runs init. This helps insure that
what is installed in the chroots is exactly what the user expects after
setting options in init.

Additionally, we create `cache_http` to verify write access to the work
folder instead of `chroot_native`. So we can ask for zapping only if
no chroot folder exists.

pmb/helpers/frontend.py

@@ -164,4 +164,4 @@ def log_distccd(args):
 
 
 def zap(args):
-    pmb.chroot.zap(args)
+    pmb.chroot.zap(args, packages=args.packages, http=args.http)