whisper: remove linter warnings (#15972)

* whisper: fixes warnings from the code linter

* whisper: more non-API-breaking changes

The remaining lint errors are because of auto-generated
files and one is because an exported function has a non-
exported return type. Changing this would break the API,
and will be part of another commit for easier reversal.

* whisper: un-export NewSentMessage to please the linter

This is an API change, which is why it's in its own commit.
This change was initiated after the linter complained that
the returned type wasn't exported. I chose to un-export
the function instead of exporting the type, because that
type is an implementation detail that I would like to
change in the near future to make the code more
readable and with an increased coverage.

* whisper: update gencodec output after upgrading it to new lint standards

whisper/whisperv6/doc.go

@@ -27,6 +27,9 @@ Whisper is a pure identity-based messaging system. Whisper provides a low-level
 or prejudiced by the low-level hardware attributes and characteristics,
 particularly the notion of singular endpoints.
 */
+
+// Contains the Whisper protocol constant definitions
+
 package whisperv6
 
 import (
@@ -34,10 +37,11 @@ import (
 	"time"
 )
 
+// Whisper protocol parameters
 const (
-	ProtocolVersion    = uint64(6)
-	ProtocolVersionStr = "6.0"
-	ProtocolName       = "shh"
+	ProtocolVersion    = uint64(6) // Protocol version number
+	ProtocolVersionStr = "6.0"     // The same, as a string
+	ProtocolName       = "shh"     // Nickname of the protocol in geth
 
 	// whisper protocol message codes, according to EIP-627
 	statusCode           = 0   // used by whisper protocol
@@ -55,7 +59,7 @@ const (
 	signatureLength = 65 // in bytes
 	aesKeyLength    = 32 // in bytes
 	AESNonceLength  = 12 // in bytes
-	keyIdSize       = 32 // in bytes
+	keyIDSize       = 32 // in bytes
 	bloomFilterSize = 64 // in bytes
 
 	EnvelopeHeaderLength = 20