The k8s-file log driver was corrupting log entries when the log file
reached max_size and needed rotation. This manifested as garbled output
with repeated timestamps and broken log entries.

The root cause was in writev_buffer_flush() which incorrectly handled
partial writes. When writev() returned a partial write, the function
would modify the original iovec base pointers, corrupting the buffer
state. During log rotation, this corrupted state would carry over to
the new file descriptor, causing subsequent log entries to be malformed.

Changes:
- Simplify writev_buffer_flush() to use individual write() calls instead
  of complex writev() partial write handling
- Always reset buffer state after log rotation to ensure clean state
  with new file descriptor
- Remove conditional buffer reset that could leave corrupted state

This fixes the issue where long-running containers (like GitLab CE)
would produce corrupted logs after reaching the configured max_size.

Fixes: Log corruption with --log-driver k8s-file --log-opt max_size=20mb

Signed-off-by: Jindrich Novy <jnovy@redhat.com>

- Add tests for log-size-max option acceptance and validation
- Test k8s-file log driver with size limits and multiple drivers
- Verify proper log file creation and format handling
- Add WithLogSizeMax() option to conmon Go API for testing

Tests ensure the writev_buffer_flush corruption fix in commit 29d17be
works correctly and log rotation doesn't corrupt k8s log entries.

Signed-off-by: Jindrich Novy <jnovy@redhat.com>

Add comprehensive test cases to validate the k8s-file log rotation fix that
prevents buffer corruption during log rotation (commit 29d17be). The original
issue manifested as corrupted log entries with repeated timestamps and broken
formatting when logs exceeded the configured max_size.

New test coverage includes:
- Small log size limit validation (50-100 bytes) to trigger rotation scenarios
- Edge case testing with various rotation thresholds (1B to 10KB)
- Log file creation and content integrity validation
- Stress testing with extremely small rotation limits
- Validation that log files can handle proper k8s format content

Signed-off-by: Jindrich Novy <jnovy@redhat.com>