Previously, newline: :lf was accepted but ignored. Where it
should have been used was commented out code that didn't work,
but unlike all other invalid values, using newline: :lf did
not raise an error.
This adds support for newline: :lf and :lf_newline, for consistency
with newline: :cr and :cr_newline. This is basically the same as
universal_newline, except that it only affects writing and not
reading due to RUBY_ECONV_NEWLINE_DECORATOR_WRITE_MASK.
Add tests for the File.open :newline option while here.
Fixes [Bug #12436]
6f3857f6a7