This change refactors package graceful into two packages: one very well
tested package that deals with graceful shutdown of arbitrary
net.Listeners in the abstract, and one less-well-tested package that
works with the nitty-gritty details of net/http and signal handling.
This is a breaking API change for advanced users of package graceful:
the WrapConn function no longer exists. This shouldn't affect most users
or use cases.