config: source job: rename field 'datasets' to 'filesystems'

While filesystems is also not the right term (since it excludes ZVOLs),
we want to stay consistent with comments & terminology used in docs.

BREAK CONFIG

fixes #17

cmd/sampleconf/pullbackup/productionhost.yml

@@ -28,7 +28,7 @@ jobs:
     client_identity: fullbackup_prod1
 
   # snapshot these filesystems every 10m with zrepl_ as prefix
-  datasets: {
+  filesystems: {
     "zroot/var/db<": "ok",
     "zroot/usr/home<": "ok",
     "zroot/var/tmp": "!", #don't backup /tmp

cmd/sampleconf/pushbackup/productionhost.yml

@@ -11,7 +11,7 @@ jobs:
       identity_file: /root/.ssh/id_ed25519
 
   # snapshot these datsets every 10m with zrepl_ as prefix
-  datasets: {
+  filesystems: {
     "zroot/var/db<": "ok",
     "zroot/usr/home<": "!",
   }

cmd/sampleconf/random/debugging.yml

@@ -22,7 +22,7 @@ jobs:
   serve:
     type: stdinserver
     client_identity: debian2
-  datasets: {
+  filesystems: {
     "pool1/db<": ok
   }
   snapshot_prefix: zrepl_