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]

Errno::EROFS may occur when the source tree is placed in a
read-only filesystem.

Paths start with the root are absolute on cygwin, regardless the
drive letter.

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.

This removes the security features added by $SAFE = 1, and warns for access
or modification of $SAFE from Ruby-level, as well as warning when calling
all public C functions related to $SAFE.

This modifies some internal functions that took a safe level argument
to no longer take the argument.

rb_require_safe now warns, rb_require_string has been added as a
version that takes a VALUE and does not warn.

One public C function that still takes a safe level argument and that
this doesn't warn for is rb_eval_cmd.  We may want to consider
adding an alternative method that does not take a safe level argument,
and warn for rb_eval_cmd.

It is said that realpath(3) and realdirpath(3) on some platforms
may return a relative path.

[Feature #15868]