Add error message for missing /etc/mtab

The zpool command should not silently fail when the /etc/mtab
file does not exist.  This can occur in an initramfs environment
when the /etc/mtab file hasn't yet been generated.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1541
Formating changes for zpool manpage

Some of these entries were hidden before.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1553
Add absent \n at the end of the help text line

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1545