[LINTER] Fix check doc incompatibility with BSD grep
[LINTER] Fix check doc incompatibility with BSD grep

Summary:
BSD grep on OSX and FreeBSD does not provide the same options as GNU
grep which prevent using them for multiline search.
This diff uses only python regex to avoid this incompatibility.
As a bonus, it runs about 3x faster.

Depends on D2694

Test Plan:
Should return no error:
  arc lint

In `src/init.cpp` delete the line 1279:
```
logger.m_print_to_console = gArgs.GetBoolArg("-printtoconsole", false);
```
Then run `arc lint` and check the linter outputs an Unknown argument
error:
  '-printtoconsole' is documented but not used'

Restore the previously deleted line in `src/init.cpp` and now delete the
lines 792 to 794:
```
strUsage += HelpMessageOpt(
        "-printtoconsole",
        _("Send trace/debug info to console instead of debug.log
file"));
```
Then run `arc lint` and check the linter outputs an Undocumented
argument error:
  '-printtoconsole' is undocumented'

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2695

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