Turns out the BC code allowed users to move from `filename` to
`template` to squash the warning without having to switch from template
paths to template contents.
Here we warn when `template` is specified as a path so we can remove the
functionality in the future and remove this source of confusion.
refs #3732
The render code path in `template_file` was doing unsynchronized access
to a shared mapping of functions in `config.Func`.
This caused a race condition that was most often triggered when a
`template_file` had a `count` of more than one, and expressed itself as
a panic in the plugin followed by a cascade of "unexpected EOF" errors
through the plugin system.
Here, we simply turn the FuncMap from shared state into a generated
value, which avoids the race. We do more re-initialization of the data
structure, but the performance implications are minimal, and we can
always revisit with a perf pass later now that the race is fixed.
Paul Hinze