brace the fact that lchmod(2) can EOPNOTSUPP

	Musl libc has this function as a tiny wrapper of fchmodat(3posix).  On
	the other hand Linux kernel does not support changing modes of a symlink.
	The operation always fails with EOPNOTSUPP.  This fchmodat behaviour is
	defined in POSIX.  We have to take care of such exceptions.
	---
	 lib/fileutils.rb               |  3 ++-
	 test/pathname/test_pathname.rb |  2 +-
	 test/ruby/test_notimp.rb       | 19 ++++++++++++-------
	 3 files changed, 15 insertions(+), 9 deletions(-)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67907 b2dd03c8-39d4-4d8f-98ff-823fe69b080e