man: change /lib to /usr/lib (#5618)

Per man:file-hierarchy(7), /lib is just a compatibility symlink; the
other manpages also refer to /usr/lib.

Found with:

    git grep -P '(?<!/usr|/var|local)/lib' man/

man/systemctl.xml

@@ -907,7 +907,7 @@ Sun 2017-02-26 20:57:49 EST  2h 3min left  Sun 2017-02-26 11:56:36 EST  6h ago
 
               <programlisting>$ systemctl status bluetooth
 ● bluetooth.service - Bluetooth service
-   Loaded: loaded (/lib/systemd/system/bluetooth.service; enabled; vendor preset: enabled)
+   Loaded: loaded (/usr/lib/systemd/system/bluetooth.service; enabled; vendor preset: enabled)
    Active: active (running) since Wed 2017-01-04 13:54:04 EST; 1 weeks 0 days ago
      Docs: man:bluetoothd(8)
  Main PID: 930 (bluetoothd)

man/systemd-delta.xml

@@ -63,7 +63,7 @@
     compare configuration files that override other configuration
     files. Files in <filename>/etc</filename> have highest priority,
     files in <filename>/run</filename> have the second highest
-    priority, …, files in <filename>/lib</filename> have lowest
+    priority, …, files in <filename>/usr/lib</filename> have lowest
     priority. Files in a directory with higher priority override files
     with the same name in directories of lower priority. In addition,
     certain configuration files can have <literal>.d</literal>

man/systemd.network.xml

@@ -1513,7 +1513,7 @@ VRF=vrf1
       <title>MacVTap</title>
       <para>This brings up a network interface <literal>macvtap-test</literal>
       and attaches it to <literal>enp0s25</literal>.</para>
-      <programlisting># /lib/systemd/network/25-macvtap.network
+      <programlisting># /usr/lib/systemd/network/25-macvtap.network
 [Match]
 Name=enp0s25