Use POSIX "df" in Tramp, if available

* lisp/net/tramp-sh.el (tramp-get-remote-df):
Support also POSIX.1 "df".
(tramp-sh-handle-file-system-info): Adapt accordingly.
2 jobs for master in 50 minutes and 8 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #603
test-all

00:50:08

passed #604
test-filenotify-gio

00:36:00