Update allowed characters; make find more selective

With percent‐encoding, it seems like the only problematic characters
are :—

- Whitespace (incompatible with make)
- Colons (incompatible with make)
- Pipes (incompatible with make as they introduce order‐only
  prerequisites)
- Bucks (incomaptible with make secondary expansion)
- Percents (incompatible with secondary expansion inside static pattern
  rules, and possibly other things)
- Leading hyphen‐minuses (confusable with a command‐line argument)

This commit updates the `find` rules to not select these files, in
addition to the existing behaviour of not selecting hidden files or
those which start with a period.

`FINDOPTS` is removed as the default is virtually always the correct
behaviour; users can override `FIND` if supplying options is absolutely
necessary.