btrbk: add system_list_mountinfo: parse /proc/self/mountinfo

preparatory patch for removing system_list_mounts().

btrbk

@@ -1760,6 +1760,53 @@ sub btrfs_send_to_file($$$;$$)
 }
 
 
+sub system_list_mountinfo($)
+{
+  my $vol = shift // die;
+  my $file = '/proc/self/mountinfo';
+  my $ret = run_cmd(cmd => [ qw(cat), $file ],
+                    rsh => vinfo_rsh($vol),
+                    non_destructive => 1,
+                    catch_stderr => 1, # hack for shell-based run_cmd()
+                   );
+  return undef unless(defined($ret));
+
+  my @mountinfo;
+  foreach (split(/\n/, $ret))
+  {
+    # https://www.kernel.org/doc/Documentation/filesystems/proc.txt
+    unless(/^(?<mount_id>[0-9]+)       # mount ID:  unique identifier of the mount (may be reused after umount)
+            \s(?<parent_id>[0-9]+)     # parent ID:  ID of parent (or of self for the top of the mount tree)
+            \s(?<st_dev>[0-9]+:[0-9]+) # major:minor:  value of st_dev for files on filesystem
+            \s(?<fs_root>\S+)          # root:  root of the mount within the filesystem
+            \s(?<mount_point>\S+)      # mount point:  mount point relative to the process's root
+            \s(?<mount_options>\S+)    # mount options:  per mount options
+            (\s\S+)*                   # optional fields:  zero or more fields of the form "tag[:value]"
+            \s-                        # separator:  marks the end of the optional fields
+            \s(?<fs_type>\S+)          # filesystem type:  name of filesystem of the form "type[.subtype]"
+            \s(?<mount_source>\S+)     # mount source:  filesystem specific information or "none"
+            \s(?<super_options>\S+)$   # super options:  per super block options
+           /x)
+    {
+      ERROR "Failed to parse \"$file\" on " . ($vol->{HOST} || "localhost");
+      DEBUG "Offending line: $_";
+      return undef;
+    }
+    my %line = %+;
+    foreach (split(',', $line{super_options})) {
+      if(/^(.+?)=(.+)$/) {
+        $line{MNTOPS}->{$1} = $2;
+      } else {
+        $line{MNTOPS}->{$_} = 1;
+      }
+    }
+    push @mountinfo, \%line;
+  }
+  # TRACE(Data::Dumper->Dump([\@mountinfo], ["mountinfo"])) if($do_dumper);
+  return \@mountinfo;
+}
+
+
 sub system_list_mounts($)
 {
   my $vol = shift // die;