Merge mkusage calls

mkusage exits by default so it's intended to exist as one call per
process only.

src/libexec/saturnin-clip

 STORAGE_DIR="$SATURNIN_CACHE_HOME/clips"
 
 usage() {
-    mkusage "[ls]"
-    mkusage "save [-1|-2|-c]"
-    mkusage "load [-1|-2|-c]"
-    mkusage "rm"
-    mkusage "clean"
+    mkusage "[ls]" \
+            "save [-1|-2|-c]" \
+            "load [-1|-2|-c]" \
+            "rm" \
+            "clean"
 }
 
 clipln() {