fixes #279

This saves about 10 seconds for a 200 switch network.

This seems slightly ugly, but it has bitten many people.

Closes #268

Add notes about specific release branches.

adjust regex for finding mininet's links during cleanup

closes #268

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()

link names of the form "a-b-ethN" were previously interpreted
as "b-ethN". this change accepts link names with a dash, and
requires N to only contain digits.

should facilitate importing 64-bit image

fixes #261