ZTS: Misc fixes for FreeBSD

• Check for mountd in is_shared to avoid timeout when not running
• Enhance robustness of some cleanup functions
• Simplify atime lookup
• Skip sharenfs validation for now
• Don't add mountpoint property to inheritance validation on FreeBSD

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10047