[SV 17448] Ignore whitespace around $(file ...) names

[SV 17448] Ignore whitespace around $(file ...) names

The other issues related to whitespace reported in this bug are
not addressed by this change.

* src/functions.c (func_file): Strip whitespace from the start and
end of the filename provided to the $(file ...) function.
* tests/scripts/functions/file: Add tests for this.
* tests/test_driver.pl: Use 3-arg version of open().