Elevate DefaultContexts() to an API function
VtdContextsDefault command
Add helper function for whenever contexts change
Context change counts as a system update
Improve performance for contexts

Split into internal-only implementations, and public interface.

The problem was that some context functions were calling others to do
some of their work, and each individually was updating the system's
contexts, which is very costly.  This caused a noticeable (multi-second)
delay when starting vim.
Set default contexts on initial run

Note that this brings a significant performance penalty -- about a half
second of startup time.  I should investigate...