* src/df.c (main): Fail and don't print the headers if no

* src/df.c (main): Fail and don't print the headers if no
file system is processed.  This makes it easy to test whether
a specified directory is on a file system of a given type or types.
Otherwise, applications would have had to parse df's output.
E.g., is "." either ext3 or reiserfs: df -t ext3 -t reiserfs .