Merge #701: Make ec_ arithmetic more consistent and add documentation

7e3952ae82af2a98619de74614f2d3f0ff072941 Clarify documentation of tweak functions. (Jonas Nick)
89853a0f2e2b324bd32e78641aeaad6d1e427e81 Make tweak function documentation more consistent. (Jonas Nick)
41fc78560223aa035d9fe2cbeedb3ee632c740e2 Make ec_privkey functions aliases for ec_seckey_negate, ec_seckey_tweak_add and ec_seckey_mul (Jonas Nick)
22911ee6da7197c45226ab4bcd84b8c2773baf9f Rename private key to secret key in public API (with the exception of function names) (Jonas Nick)
5a73f14d6cfab043f94b2032d6958e15f84065fe Mention that value is unspecified for In/Out parameters if the function returns 0 (Jonas Nick)
f03df0e6d7d272808c9d27df40bb92716b341d03 Define valid ECDSA keys in the documentation of seckey_verify (Jonas Nick)
5894e1f1df74b23435cfd1e9a8b25f22ac631ebe Return 0 if the given seckey is invalid in privkey_negate, privkey_tweak_add and privkey_tweak_mul (Jonas Nick)
8f814cddb94f4018646569143d10ebbb98daa454 Add test for boundary conditions of scalar_set_b32 with respect to overflows (Jonas Nick)
3fec9826086aa45ebbac1ff6fc3bb7b25ca78b1d Use scalar_set_b32_seckey in ecdsa_sign, pubkey_create and seckey_verify (Jonas Nick)
9ab2cbe0ebf8dfafd5499ea7a79c35f0d0cfe5e2 Add scalar_set_b32_seckey which does the same as scalar_set_b32 and also returns whether it's a valid secret key (Jonas Nick)

Pull request description:

  Fixes #671. Supersedes #668.

  This PR unifies handling of invalid secret keys by introducing a new function `scalar_set_b32_secret` which returns false if the b32 overflows or is 0. By using this in `privkey_{negate, tweak_add, tweak_mul}` these function will now return 0 if the secret key is invalid which matches the behavior of `ecdsa_sign` and `pubkey_create`.

  Instead of deciding whether to zeroize the secret key on failure, I only added documentation for now that the value is undefined on failure.

ACKs for top commit:
  real-or-random:
    ACK 7e3952ae82af2a98619de74614f2d3f0ff072941 I read the diff carefully and tested the changes
  apoelstra:
    ACK 7e3952ae82af2a98619de74614f2d3f0ff072941

Tree-SHA512: 8e9a66799cd3b6ec1c3acb731d6778035417e3dca9300d840e2437346ff0ac94f0c9be4de20aa2fac9bb4ae2f8a36d4e6a34795a640b9cfbfee8311decb102f0