Close the file when done with it.

fixup: useless_parenthesis

Add spaces.

OK...

print error messages when making interfaces

userspace switch can be reached on unix port by dpctl

This is added because we install Ubuntu from a serial console.
Fixes #265

don't crash when link is TCLink but without options

fixup: unresolved_references

without this, thereis no indication in Mininet that creating an
interface has failed. for example, this may happen when the interface
name is too long (longer than IFNAMSIZ, which is 16 chars).

caused by optimized return in link.py#275 in TCIntf.config()

should facilitate importing 64-bit image

fixes #261

kill entire process group in Node.terminate

mnexec already puts the shell into its own process group. Killing the entire
process group cleans up after any background processes the user left running.