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

tests/zfs-tests/tests/functional/inheritance/inherit_001_pos.ksh

@@ -406,14 +406,15 @@ if is_linux; then
 	def_val+=("off")
 	local_val+=("off")
 else
-	prop+=("aclmode" "" \
-		"mountpoint" "")
-	def_val+=("discard" \
-		"")
-	local_val+=("groupmask" \
-		"$TESTDIR")
+	prop+=("aclmode" "")
+	def_val+=("discard")
+	local_val+=("groupmask")
+fi
+if is_illumos; then
+	prop+=("mountpoint" "")
+	def_val+=("")
+	local_val+=("$TESTDIR")
 fi
-
 
 #
 # Global flag indicating whether the default record size had been