Fix issues in pass_util.cc password handling

[justsmth]

Description of changes:

Addresses multiple issues in tool-openssl/pass_util.cc and the Password class:

• Improved secure memory clearing to cover residual bytes left by string mutations, and ensured all temporary buffers holding passwords are properly cleansed.
• Fixed fd: parsing to use strtoul instead of atoi, avoiding undefined behavior on overflow.
• Removed incorrect rejection of colons in pass: passwords, matching OpenSSL behavior.
• Fixed off-by-one in password length limits for pass: and env: sources.
• Minor hardening: removed env var names from error messages, fixed same_file initialization.

Call-outs:

• Behavioral change: pass:a:b was previously rejected and now succeeds with password a:b.
• Password::secure_clear() now cleanses up to capacity() bytes, which is technically beyond the std::string standard contract but safe across all major implementations.

Testing:

All existing PassUtilTest tests pass. Updated DirectPasswordEdgeCases to cover passwords containing colons.

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 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.51%. Comparing base (0628190) to head (a9dc9a4).

Files with missing lines	Patch %	Lines
tool-openssl/pass_util.cc	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3032      +/-   ##
==========================================
+ Coverage   78.35%   78.51%   +0.16%     
==========================================
  Files         689      689              
  Lines      121010   121014       +4     
  Branches    16992    16995       +3     
==========================================
+ Hits        94813    95016     +203     
+ Misses      25302    25099     -203     
- Partials      895      899       +4     

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