Make use of constant time ecdsa_sign code in ecdsa_sign_recoverable

[jonasnick]

We can not add secp256k1_ecdsa_sign_recoverable to valgrind_ctime_tests (added in PR #708) because it is mostly a copy of the old ecdsa_sign function which was not constant time in edge cases. Instead of fixing sign_recoverable and again having duplicate code we should add an internal function with the code shared between the two functions (similar to ab4fd52).

Let's get #701 in first because it also touches ecdsa_sign.

[elichai]

@jonasnick hehe I didn't know this issue existed :)