Make ec_ arithmetic more consistent and add documentation

Summary:

• Add scalar_set_b32_seckey which does the same as scalar_set_b32 and also returns whether it's a valid secret key
• Use scalar_set_b32_seckey in ecdsa_sign, pubkey_create and seckey_verify
• Add test for boundary conditions of scalar_set_b32 with respect to overflows
• Return 0 if the given seckey is invalid in privkey_negate, privkey_tweak_add and privkey_tweak_mul
• Define valid ECDSA keys in the documentation of seckey_verify
• Mention that value is unspecified for In/Out parameters if the function returns 0
• Rename private key to secret key in public API (with the exception of function names)
• Make ec_privkey functions aliases for ec_seckey_negate, ec_seckey_tweak_add and ec_seckey_mul
• Make tweak function documentation more consistent.

Do this by adding a newline after the first sentence and aligning the rest.

• Clarify documentation of tweak functions.

In particular, mention that the functions return 0 if seckey or tweak are
invalid (as opposed to saying "should" or "must" be valid).

This is a backport of libsecp256k1 PR701

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7587