mount has the following exit status values (the bits can be
ORed):
0
success
1
incorrect invocation or permissions
2
system error (out of memory, cannot fork, no more loop
devices)
4
internal mount bug
8
user interrupt
16
problems writing or locking /etc/mtab
32
mount failure
64
some mount succeeded
The command mount -a returns 0 (all succeeded), 32 (all
failed), or 64 (some failed, some succeeded).
