minijail: Add named constants for seccomp filters

This makes it possible to write filters using named constants (like
ENOSYS instead of 38).

BUG=chromium:516701
TEST=syscall_filter_unittest passes.

Change-Id: Ic44cbdfb6f2228f6f658b1cc48adf5a923394306
Reviewed-on: https://chromium-review.googlesource.com/290540
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Trybot-Ready: Luis Hector Chavez <lhchavez@google.com>
Tested-by: Luis Hector Chavez <lhchavez@google.com>
Commit-Queue: Luis Hector Chavez <lhchavez@google.com>
diff --git a/util.h b/util.h
index d07e5b1..a88cbc8 100644
--- a/util.h
+++ b/util.h
@@ -31,6 +31,7 @@
 
 int lookup_syscall(const char *name);
 const char *lookup_syscall_name(int nr);
+long int parse_constant(char *constant_str, char **endptr);
 char *strip(char *s);
 char *tokenize(char **stringp, const char *delim);