Re-enable lint checks

[cmihai]

This PR tries to fix and re-enable the lint checks that were disabled for the 0.17 merge. Enabled checks:

• Git subtree checks for src/crypto/ctaes, src/secp256k1, src/univalue and src/leveldb
• lint-circular-dependencies.sh, lint-filenames.sh, lint-format-strings.sh, lint-includes.sh, lint-locale-dependence.sh, lint-logs.sh, lint-python-shebang.sh, lint-python-utf8-encoding.sh, lint-shell-locale.sh, lint-whitespace.sh

The last several checks require extensive code changes, so I'd like to submit them in separate PRs.

Part of #1054.

[scravy]

Concept ACK

[scravy]

A linting step failed, probably due to rebase with https://github.com/dtr-org/unit-e/pull/1080

Had this pull request been merged before mine I'd have had to justfiy introducing a boost dependency, so let me speak here ;-)

This is a dependency which is only introduced in tests + we do not have any intention to move away from the boost unit tests. While in general the idea is to migrate boost-features to equivalent C++ native features (like favor std::option over boost::optional, not yet available in C++11 though), we are not discouraging reusability and proper software engineering. The dependency is not exactly new as in it already is available to us and it's actually required to use a piece of functionality which boost unit test already provides (and we are using the boost unit test framework after all).

So I guess it should be added to the known boost dependencies.