filesystem: implement '-P --portability' option for df
 * suggested by tomas-edwardsson on github
 * retires need for parsing filesystems over multiple lines
 * also fixes for darwin and other non-linux kernels
Revert "filesystem: fix bug due to filesystems spanning multiple lines"
 * Hack is no longer necessary because -P option has been added to df.

This reverts commit a7f242266c7e5e5aafc1ee2c2f5a6dd350ca6ff4.