all: Fix some linter errors (#5499)

I'm working through linter complaints, these are some fixes. Broad
categories:

1) Ignore errors where we can ignore errors: add "_ = ..." construct.
you can argue that this is annoying noise, but apart from silencing the
linter it *does* serve the purpose of highlighting that an error is
being ignored. I think this is OK, because the linter highlighted some
error cases I wasn't aware of (starting CPU profiles, for example).

2) Untyped constants where we though we had set the type.

3) A real bug where we ineffectually assigned to a shadowed err.

4) Some dead code removed.

There'll be more of these, because not all packages are fixed, but the
diff was already large enough.

lib/upgrade/upgrade_common.go

@@ -83,10 +83,10 @@ type Relation int
 
 const (
 	MajorOlder Relation = -2 // Older by a major version (x in x.y.z or 0.x.y).
-	Older               = -1 // Older by a minor version (y or z in x.y.z, or y in 0.x.y)
-	Equal               = 0  // Versions are semantically equal
-	Newer               = 1  // Newer by a minor version (y or z in x.y.z, or y in 0.x.y)
-	MajorNewer          = 2  // Newer by a major version (x in x.y.z or 0.x.y).
+	Older      Relation = -1 // Older by a minor version (y or z in x.y.z, or y in 0.x.y)
+	Equal      Relation = 0  // Versions are semantically equal
+	Newer      Relation = 1  // Newer by a minor version (y or z in x.y.z, or y in 0.x.y)
+	MajorNewer Relation = 2  // Newer by a major version (x in x.y.z or 0.x.y).
 )
 
 // CompareVersions returns a relation describing how a compares to b.