Add truncate: to StandardOutput= etc.

This adds the ability to specify truncate:PATH for StandardOutput= and
StandardError=, similar to the existing append:PATH. The code is mostly
copied from the related append: code. Fixes #8983.

test/units/testsuite-27.sh

@@ -43,6 +43,18 @@ a
 c
 EOF
 
+systemd-run --wait --unit=test27-four \
+            -p StandardOutput=truncate:/tmp/stdout \
+            -p StandardError=truncate:/tmp/stderr \
+            -p Type=exec \
+            sh -c 'echo a ; echo b >&2'
+cmp /tmp/stdout <<EOF
+a
+EOF
+cmp /tmp/stderr <<EOF
+b
+EOF
+
 systemd-analyze log-level info
 
 echo OK >/testok