err: Add err(), warn(), errx() and warnx() familiy of functions

Some systems such as Windows or musl-libc based ones do not have these
BSD extensions. In addition libbsd itself is making use of the warnx()
functions, so we better provide these interfaces in case they are
missing.

src/libbsd.map

@@ -168,3 +168,16 @@ LIBBSD_0.9.1 {
     strnvis_netbsd;
     strnunvis_netbsd;
 } LIBBSD_0.9;
+
+LIBBSD_0.10.0 {
+    /* These BSD extensions are available on GNU systems, but not on other
+     * systems such as Windows or musl libc based ones. */
+    vwarn;
+    vwarnx;
+    warn;
+    warnx;
+    verr;
+    verrx;
+    err;
+    errx;
+} LIBBSD_0.9.1;