termios: add tcflow() and underlying TCXONC ioctl

[julianuziemblo]

YT: RTOS-1355

Description

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[gemini-code-assist]

Code Review

This pull request introduces the tcflow function and the TCXONC ioctl definition to the termios library. The implementation of tcflow correctly handles system call interruptions by retrying on EINTR. There are no review comments, and I have no feedback to provide.

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[github-actions]

Unit Test Results

10 860 tests  ±0   10 190 ✅ ±0   52m 47s ⏱️ -11s
   670 suites ±0      670 💤 ±0 
     1 files   ±0        0 ❌ ±0 

Results for commit c7f7657. ± Comparison against base commit fb9670d.

♻️ This comment has been updated with latest results.