Put modification times in the gzip header only for regular files.

This replicates the behavior of gzip since version 1.10. The intent
is to make piped applications of pigz produce deterministic output.
Note that input redirected from a regular file is still a regular
file.

pigz.c

@@ -3922,8 +3922,8 @@ local void process(char *path) {
         vstrcpy(&g.inf, &g.inz, 0, "<stdin>");
         g.ind = 0;
         g.name = NULL;
-        g.mtime = g.headis & 2 ?
-                  (fstat(g.ind, &st) ? time(NULL) : st.st_mtime) : 0;
+        g.mtime = (g.headis & 2) && fstat(g.ind, &st) == 0 &&
+                  S_ISREG(st.st_mode) ? st.st_mtime : 0;
         len = 0;
     }
     else {