Revert "Revert "filesystem: fix bug due to filesystems spanning multiple lines""
 * It turns out that darwin/xserve STILL returns output
   spanning multiple lines with the -P option enabled.

This reverts commit 46df3668eb4e1ade4274d86211874a643aaf4e6f.
test: add test for hpux
fix: catch errors parsing df output on darwin, and regression test
fix: naga return code was always 0
update README