(maint) Refactor aix mountpoints reading

Previously, the AIX mountpoint resolver would check each line of the
`mount` command output to check if it matches the header of the table,
so it can be skipped.

This is not efficient and it can cause many problems, such as not
skipping the first line of the output if spaces are not present before
`node`, therefore considering the header of the table as a mountpoint.

```
  node       mounted        mounted over    vfs       date        options
-------- ---------------  ---------------  ------ ------------ ---------------
         /dev/hd4         /                jfs2   Mar 22 08:05 rw,log=/dev/hd8
         /dev/hd2         /usr             jfs2   Mar 22 08:05 rw,log=/dev/hd8
         /proc            /proc            procfs Mar 22 08:05 rw
         /dev/hd10opt     /opt             jfs2   Mar 22 08:05 rw,log=/dev/hd8
         /dev/hd3         /var             x      Mar 22 08:05 rw,nodev,log=/dev/hd3
```

To fix this, instead of checking a line matches the header, we just
simply skip the first 2 lines of the output, removing unnecessary regex
expresions and improving performance

lib/facter/resolvers/aix/mountpoints.rb

@@ -18,23 +18,26 @@ def post_resolve(fact_name, _options)
           def read_mount(fact_name)
             @fact_list[:mountpoints] = {}
             output = Facter::Core::Execution.execute('mount', logger: log)
-            output.split("\n").map do |line|
-              next if line =~ /\s+node\s+|---|procfs|ahafs/
+            output.split("\n").drop(2).map do |line|
+              next if line =~ /procfs|ahafs/
 
-              elem = line.split("\s")
-
-              @fact_list[:mountpoints][elem[1]] = { device: elem[0], filesystem: elem[2],
-                                                    options: elem.last.split(',') }
+              add_mount_points_fact(line)
             end
 
             retrieve_sizes_for_mounts
             @fact_list[fact_name]
           end
 
+          def add_mount_points_fact(line)
+            elem = line.split("\s")
+            @fact_list[:mountpoints][elem[1]] = { device: elem[0], filesystem: elem[2],
+                                                  options: elem.last.split(',') }
+          end
+
           def retrieve_sizes_for_mounts
             output = Facter::Core::Execution.execute('df -P', logger: log)
-            output.split("\n").map do |line|
-              next if line =~ /Filesystem|-\s+-\s+-/
+            output.split("\n").drop(1).map do |line|
+              next if line =~ /-\s+-\s+-/
 
               mount_info = line.split("\s")
               mount_info[3] = translate_to_bytes(mount_info[3])