Restore log_prefix

The slave is supposed to prefix its diagnostics with its host name. This
is why warn() and die() support a log_prefix.

However, the code to actually set the log prefix must have been lost.

Set log_prefix to hostname on slave.

cmirror.c

@@ -653,6 +653,8 @@ static void slave(char *slave_path) {
     FILE *in  = stdin;
     FILE *out = stdout;
 
+    log_prefix = g_get_host_name();
+
     EXCEPTS = g_ptr_array_new();
     CLEAN = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL);
 
@@ -736,7 +738,7 @@ static void slave(char *slave_path) {
             continue;
         }
         if (words != 9)
-            die("%s: wrong number of words in index record from master: %s\n", g_get_host_name(), line);
+            die("wrong number of words in index record from master: %s\n", line);
 
         g_autoptr(FileInfo) fi_want = fileinfo_new_import(word);
         char *filename = fi_want->name;