Since chroot() doesn't chdir() by default, we are left in
an unreachable directory in node.pexec() (and in xterms.)

fixes #370

fixes #347

added persistence option to HostWithPrivateDirs. also attached mount namespaces when mnexec -a is specified

The sysfs filesystem is [tagged][1] with a set of namespaces when mounted, taken
from the mounting process. Among other things, this controls which network
devices will show up in /sys/class/net and /sys/class/net/bonding_masters.

Without this change, mininet will not mount sysfs in a node. Attempting to
configure a bond interface in a node will only affect the parent namespace.

This change mounts a new sysfs filesystem in each node. To prevent this mount
from affecting the parent namespace the mount namespace is also unshared.

[1]: https://www.kernel.org/doc/Documentation/filesystems/sysfs-tagging.txt

getopt returns an 'int', so use change datatype of 'c' to match it.
Otherwise, 'c' may hold a value of 255 (0xff), and fail the comparison
with -1 (0xffffffff): while ((c = getopt(...)) != -1)

This bug was uncovered on Ubuntu 13.04 running on ARM, using
arm-linux-gnueabihf-gcc4.7.

fixes #127

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.

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.