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/message.go

@@ -33,7 +33,8 @@ import (
 	"github.com/ethereum/go-ethereum/log"
 )
 
-// Options specifies the exact way a message should be wrapped into an Envelope.
+// MessageParams specifies the exact way a message should be wrapped
+// into an Envelope.
 type MessageParams struct {
 	TTL      uint32
 	Src      *ecdsa.PrivateKey
@@ -86,8 +87,8 @@ func (msg *ReceivedMessage) isAsymmetricEncryption() bool {
 	return msg.Dst != nil
 }
 
-// NewMessage creates and initializes a non-signed, non-encrypted Whisper message.
-func NewSentMessage(params *MessageParams) (*sentMessage, error) {
+// NewSentMessage creates and initializes a non-signed, non-encrypted Whisper message.
+func newSentMessage(params *MessageParams) (*sentMessage, error) {
 	msg := sentMessage{}
 	msg.Raw = make([]byte, 1, len(params.Payload)+len(params.Padding)+signatureLength+padSizeLimit)
 	msg.Raw[0] = 0 // set all the flags to zero
@@ -341,7 +342,8 @@ func (msg *ReceivedMessage) extractPadding(end int) (int, bool) {
 	return paddingSize, true
 }
 
-// Recover retrieves the public key of the message signer.
+// SigToPubKey returns the public key associated to the message's
+// signature.
 func (msg *ReceivedMessage) SigToPubKey() *ecdsa.PublicKey {
 	defer func() { recover() }() // in case of invalid signature