Support for control-C. Finally.

I've changed the way things work a bit:

1. netns is replaced by mnexec, a general-purpose mininet helper.

2. For interactive commands, we now use mnexec -p, which prints out
   the pid, so we can kill it when someone hits control-C!

3. We close file descriptors for subshells. This might save memory,
   but who knows.

4. We detach our subshells from the tty using mnexec -s; thus
   control-C should not terminate everything.

5. Given 4, mn -c is now necessary if you kill mininet.

mnexec.c

+/* mnexec: execution utility for mininet
+ *
+ * Starts up programs and does things that are slow or
+ * difficult in Python, including:
+ *
+ *  - closing all file descriptors except stdin/out/error
+ *  - detaching from a controlling tty using setsid
+ *  - running in a network namespace
+ *  - printing out the pid of a process so we can identify it later
+ *
+ * Partially based on public domain setsid(1)
+*/
+
+#include <stdio.h>
+#include <linux/sched.h>
+#include <unistd.h>
+
+void usage(char *name) 
+{
+    printf("Execution utility for Mininet.\n"
+           "usage: %s [-cdnp]\n"
+           "-c: close all file descriptors except stdin/out/error\n"
+           "-d: detach from tty by calling setsid()\n"
+           "-n: run in new network namespace\n"
+           "-p: print ^A + pid\n", name);
+}
+
+int main(int argc, char *argv[])
+{
+    char c;
+    int fd;
+    int opt;
+    
+    while ((c = getopt(argc, argv, "+cdnp")) != -1)
+        switch(c) {
+        case 'c':
+            /* close file descriptors except stdin/out/error */
+            for (fd = getdtablesize(); fd > 2; fd--)
+                close(fd);
+            break;
+        case 'd':
+            /* detach from tty */
+            if (getpgrp() == getpid()) {
+                switch(fork()) {
+                    case -1:
+                        perror("fork");
+                        return 1;
+                    case 0:		/* child */
+                        break;
+                    default:	/* parent */
+                        return 0;
+                }
+            }
+            setsid();
+            break;
+        case 'n':
+            /* run in network namespace */
+            if (unshare(CLONE_NEWNET) == -1) {
+                perror("unshare");
+                return 1;
+            }
+            break;
+        case 'p':
+            /* print pid */
+            printf("\001%d\n", getpid());
+            fflush(stdout);
+            break;
+        default:
+            usage(argv[0]);
+            break;
+        }
+
+    if (optind < argc) {
+		execvp(argv[optind], &argv[optind]);
+		perror("execvp");
+		return 1;
+	}
+    
+    usage(argv[0]);
+}