implement --state exists

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>

conf/type/__file/gencode-local

@@ -23,6 +23,9 @@
 
 destination="/$__object_id"
 state_should="$(cat "$__object/parameter/state")"
+exists="$(cat "$__object/explorer/exists")"
+
+[ "$state_should" = "exists" -a "$exists" = "yes" ] && exit 0 # nothing to do
 
 if [ "$state_should" = "present" ]; then
    if [ -f "$__object/parameter/source" ]; then

conf/type/__file/man.text

@@ -21,7 +21,11 @@ None.
 OPTIONAL PARAMETERS
 -------------------
 state::
-   'present' or 'absent', defaults to 'present'
+   'present', 'absent' or 'exists', defaults to 'present'
+   where:
+      present: the file is exactly the one from source
+      absent: the file does not exist
+      exists: the file from source but only if it doesn't already exist
 
 group::
    Group to chgrp to.