man: be more explicit about thread safety of sd_journal

Triggered by https://bugzilla.redhat.com/show_bug.cgi?id=1609349

This adds two generic paragaphs we include via xinclude. One is the
"strict" version, which contains wording saying that we are thread
agnostic and what that means. And the other is the "safe" version, for
the cases we provide fully safety.

Let's then change most man pages to use either of these generic
paragraphs. With one exception: man/sd_journal_get_catalog.xml contains
both kinds of function, we hence use manual wording.

man/libudev.xml

@@ -48,11 +48,9 @@
     <citerefentry><refentrytitle>udev_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
     It is used to track library state and link objects together. No
     global state is used by libudev, everything is always linked to
-    a udev context. Furthermore, multiple different udev contexts can
-    be used in parallel by multiple threads. However, a single context
-    must not be accessed by multiple threads in parallel. The caller
-    is responsible for providing suitable locking if they intend to use
-    it from multiple threads.</para>
+    a udev context.</para>
+
+    <xi:include href="threads-aware.xml" xpointer="strict"/>
 
     <para>To introspect a local device on a system, a udev device
     object can be created via