crypto/ssh: Parse ECDSA key using struct
crypto/ssh: Parse ECDSA key using struct

Change parseECDSA() to unmarshal the key's contents into a struct
representing the wire format, consistent with the parseRSA() and
parseDSA(), to make the code more readable and its intent clearer.

Change-Id: Iea85630107ac0b3e681807d2278390c8c50ce141
Reviewed-on: https://go-review.googlesource.com/13663
Reviewed-by: Han-Wen Nienhuys <80fd3ae711f32de87aab2cdfbc261d7054711dd4@gmail.com>
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

Carrotman42/crypto,ScriptRock/crypto,pki-io/crypto,thomaso-mirodin/crypto,kim-racktop/crypto,Carrotman42/crypto,cmars/tools,bford/golang-x-crypto,golang/crypto,golang/crypto,pki-io/crypto,cmars/tools,ScriptRock/crypto,kim-racktop/crypto,golang/crypto,mattharden/crypto,cmars/tools,thomaso-mirodin/crypto,cmars/tools,mattharden/crypto,bford/golang-x-crypto