Harden CLI input validation and file output

[nhatnghiho]

Issues:

Resolves P469904855

Description of changes:

Fixes input validation gaps and tightens file permissions in the CLI tool:

• Always fail on DNS resolution errors in s_client regardless of quiet mode
• Reject unrecognized -v2 cipher names in pkcs8 -topk8 instead of falling through
• Set restrictive permissions (0600) when writing private key files

Testing:

Tests added for each change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.92308% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.16%. Comparing base (13fdb9e) to head (5799c10).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
tool/file.cc	0.00%	18 Missing ⚠️
tool/transport_common.cc	20.00%	2 Missing and 2 partials ⚠️
tool-openssl/ec_test.cc	75.00%	1 Missing ⚠️
tool-openssl/pkcs8_test.cc	88.88%	1 Missing ⚠️
tool-openssl/pkey_test.cc	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3347      +/-   ##
==========================================
- Coverage   78.18%   78.16%   -0.02%     
==========================================
  Files         693      693              
  Lines      123959   124004      +45     
  Branches    17213    17217       +4     
==========================================
+ Hits        96912    96929      +17     
- Misses      26129    26154      +25     
- Partials      918      921       +3     

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[justsmth]

Build failure on some platforms:

/Users/runner/work/aws-lc/aws-lc/third_party/googletest/include/gtest/gtest.h:1545:11: error: comparison of integers of different signs: 'const int' and 'const unsigned int' 
...
/Users/runner/work/aws-lc/aws-lc/tool-openssl/ec_test.cc:238:3: note: in instantiation of function template specialization 'testing::internal::EqHelper::Compare<int, unsigned int, nullptr>' requested here
  EXPECT_EQ(perms & 0077, 0u)
  ^
...                                         ^
1 error generated.