Do not allow 64 bit ciphers for encryption without explicit option.

[desvxx]

fixes #1598

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.15068% with 5 lines in your changes missing coverage. Please review.

Project coverage is 84.82%. Comparing base (3bd1b71) to head (68f103b).

Files with missing lines	Patch %	Lines
src/lib/rnp.cpp	92.30%	2 Missing :warning:
src/rnp/fficli.cpp	92.30%	2 Missing :warning:
src/rnp/rnpcfg.cpp	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2266 +/- ## ========================================== + Coverage 84.81% 84.82% +0.01% ========================================== Files 116 116 Lines 23292 23358 +66 ========================================== + Hits 19755 19814 +59 - Misses 3537 3544 +7 ```

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[ni4]

@desvxx linter failed.