logging: mention -v and rename log file

Previously, the log file written to by fvwm3 used to contain the pid in
the file name.  This isn't useful, and since the log file is opened in
append mode anyway, it makes more sense to remove this.

Also added a description to the man page.

Helps with fvwmorg#77

doc/fvwm/options.xml

@@ -367,5 +367,11 @@ internal debugging and should only be used by developers.</para>
 
   </listitem>
   </varlistentry>
+  <varlistentry>
+  <term><option>-v</option></term>
+  <listitem>
+<para>Enables debug logging.  Write to ~/fvwm3-output.log in append mode.</para>
+  </listitem>
+  </varlistentry>
 </variablelist>
 </section>

libs/log.c

@@ -46,15 +46,13 @@ log_set_level(int ll)
 void
 log_open(void)
 {
-	char	*path;
+	char	*path = "fvwm3-output.log";
 
 	if (log_level == 0)
 		return;
 	log_close();
 
-	xasprintf(&path, "fvwm3-%ld.log", (long)getpid());
 	log_file = fopen(path, "a");
-	free(path);
 	if (log_file == NULL)
 		return;