all: fix errors reported by vet, golint

None are "wrong" per se, but there are a lot of good suggestions and
in one case a docstring that was not present in godoc due to the
presence of an extra newline.

Changed "Id" in struct properties to "ID" in some non-exported
structs. Removed a trailing period from some error messages; I believe
the exact contents of error strings are not covered by the Go
compatibility promise.

Change-Id: I7c620582dc247396f72c52d38c909ccc0ec87b83
Reviewed-on: https://go-review.googlesource.com/80145
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/knownhosts/knownhosts.go

@@ -108,8 +108,8 @@ func wildcardMatch(pat []byte, str []byte) bool {
 	}
 }
 
-func (l *hostPattern) match(a addr) bool {
-	return wildcardMatch([]byte(l.addr.host), []byte(a.host)) && l.addr.port == a.port
+func (p *hostPattern) match(a addr) bool {
+	return wildcardMatch([]byte(p.addr.host), []byte(a.host)) && p.addr.port == a.port
 }
 
 type keyDBLine struct {