add make target to build static binary

This is useful to generate a binary that can be dropped
onto any arbitrary distro, such as CoreOS, without having
to worry about glibc or other dependencies.

Specifically: CoreOS uses glibc, but it does not have a
package manager. It also has a read-only OS (`/usr/`).
Thus I'd like to compile a binary that can be dropped
into CoreOS and have zero dependencies.

* `make build` builds the same as it did before this commit.
* `make all` builds both dynamic and static bins, as expected.

I struggled with a way to DRY this up _and_ avoid diff churn.
In the end, I went with simplicity even though it's not DRY.

.gitignore

@@ -6,4 +6,5 @@ old/
 tmp/
 *_stringer.go
 mgmt
+mgmt.static
 rpmbuild/