Set default PROG in bash_autocomplete

By setting the default value of PROG to the basname of whatever the
filename is, we allow bash_autocomplete to be copied into
/etc/bash_completion.d without modification.

autocomplete/bash_autocomplete

@@ -1,5 +1,7 @@
 #! /bin/bash
 
+: ${PROG:=$(basename ${BASH_SOURCE})}
+
 _cli_bash_autocomplete() {
      local cur prev opts base
      COMPREPLY=()