cp: fix --preserve=ownership permissions bug

cp: fix --preserve=ownership permissions bug

This fixes a bug that I introduced in
2006-12-06T19:44:08Z!eggert@cs.ucla.edu.
* src/copy.c (USE_XATTR): New macro.
(copy_reg): Use it to help the compiler.  Prefer open u+w to a
later chmod u=rw; u+r isn’t needed for xattr.  For the later u-r,
do only one (or zero) chmod calls instead of two (or one).
In the last chmod, respect the umask instead of ignoring it.
* tests/cp/preserve-mode.sh: Test for the bug.