mount: (new) cleanup mount -a return codes

mount: (new) cleanup mount -a return codes

New return codes:

  0 : all mounted (or all ignored)
 64 : some mounted, some failed
 32 : all failed

Note that already mounted or ignored (filtered out by -t or -O)
filesystems don't affect the final return code.

The original mount(8) returns 0 instead of 64, so the situation
"some mounted, some failed" cannot be detected.

Signed-off-by: Karel Zak <kzak@redhat.com>