df: fix exit code and error messages with --total

df: fix exit code and error messages with --total

When the combination of the file system options with given files or
devices does not lead to output, "df --total" would exit successfully
although it should not.

Examples:
  $ df --total --type=xfs /          # when / is not an XFS file system
  $ df --total --local -t nfs  DIR   # nfs is remote per se ...
  $ df --total -t qwerty /dev/sdb5   # typo in file system type

Furthermore, "df --total" would not print the error message "no file
systems processed" when the file argument does not exist or is otherwise
not accessible.

Example:
  $ df --total __not_exist__

These 2 bugs are present since --total was added by commit
v6.12-166-gea2887b.

* src/df.c (get_dev): Do not set file_systems_processed to true when
force_fsu is true, i.e. when the row for the "total" line is processed.
(main): Don't print totals unless we've processed a file system.
Also only print the "no FS processed" message if there was no
preceding diagnostic.
* tests/df/total-unprocessed: Add a new test.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the fix.

Improved-by: Jim Meyering