name the /proc mount

The current mount code ends up generating a mount point like:
 /proc proc ro,nosuid,nodev,noexec,relatime 0 0

This confuses `mount` and users.  Give it a standard name of "proc".

Bug: None
Test: `mount` inside of minijail looks sane now
Change-Id: I771a32eea340cfe0a6bc9d21520057e0491de9f4
diff --git a/libminijail.c b/libminijail.c
index f93c834..c0fd59e 100644
--- a/libminijail.c
+++ b/libminijail.c
@@ -1264,7 +1264,7 @@
 			return -errno;
 		}
 	}
-	if (mount("", kProcPath, "proc", kSafeFlags | MS_RDONLY, ""))
+	if (mount("proc", kProcPath, "proc", kSafeFlags | MS_RDONLY, ""))
 		return -errno;
 	return 0;
 }