ssh_filter_btrbk: correctly handle --sudo option

ChangeLog

@@ -1,6 +1,8 @@
 btrbk-current
 
   * MIGRATION
+    - If you call ssh_filter_btrbk.sh with "--sudo" option, make sure
+      to set "backend btrfs-progs-sudo" in btrbk.conf for this host.
     - If "rate_limit" is enabled, update ssh_filter_btrbk.sh on remote
       source hosts, and make sure the "pv" command is available there.
   * Allow converting backup disks to source disks (close #114).

doc/ssh_filter_btrbk.1

@@ -89,7 +89,8 @@ Log ACCEPT and REJECT messages to the system log.
 .PP
 \-\-sudo
 .RS 4
-Call SSH_ORIGINAL_COMMAND using sudo.
+Allow btrfs commands to be called via sudo. Enable this if you have
+"backend btrfs-progs-sudo" in your btrbk configuration file.
 .RE
 .SH AVAILABILITY
 Please refer to the btrbk project page \fBhttp://digint.ch/btrbk/\fR

ssh_filter_btrbk.sh

@@ -89,8 +89,12 @@ reject_filtered_cmd()
 }
 
 
-# FIXME: "--sudo" must be set before all other options!
+# check for "--sudo" option before processing other options
 sudo_prefix=
+for key; do
+    [[ "$key" == "--sudo" ]] && sudo_prefix="sudo -n "
+done
+
 while [[ "$#" -ge 1 ]]; do
     key="$1"
 
@@ -100,7 +104,7 @@ while [[ "$#" -ge 1 ]]; do
           ;;
 
       --sudo)
-          sudo_prefix="sudo -n "
+          # already processed above
           ;;
 
       -p|--restrict-path)