use helpers from c.h

A few files still use the C library helpers from err.h, so migrate
them to the local c.h helpers.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

proc/sig.c

@@ -13,6 +13,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include "sig.h"
+#include "c.h"
 
 /* Linux signals:
  *
@@ -221,7 +222,7 @@ char *strtosig(const char *restrict s){
 
   copy = strdup(s);
   if (!copy)
-    err(EXIT_FAILURE, "cannot duplicate string");
+    xerrx(EXIT_FAILURE, "cannot duplicate string");
   for (p = copy; *p != '\0'; p++)
     *p = toupper(*p);
   p = copy;