libcrun: fall back to bind mount for /dev/console on read-only file systems

[jnovy]

Commit 405d2a2 changed /dev/console setup to use unlink+symlink instead
of a bind mount when mount_dev_from_host is false. When the rootfs is
read-only and /dev is not a separate tmpfs mount, unlink fails with EROFS.

Fall back to the original bind mount approach when unlink returns EROFS.

Closes #1745

[gemini-code-assist]

Summary of Changes

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This pull request addresses an issue where container terminal setup would fail on read-only root filesystems if /dev was not a separate tmpfs. Previously, the system attempted to unlink and symlink /dev/console, which would fail with EROFS on a read-only filesystem. The changes introduce a fallback to the original bind mount approach for /dev/console specifically when an EROFS error is encountered during the unlink operation, ensuring robust terminal allocation in such environments. A new test case has been added to validate this fix.

Highlights

• Error Handling: Implemented a fallback mechanism for /dev/console setup when unlink fails due to a read-only filesystem (EROFS).
• Terminal Setup: Reverted to using a bind mount for /dev/console in read-only rootfs scenarios where /dev is not a separate tmpfs.
• Testing: Added a new test case to verify terminal functionality on read-only root filesystems without a dedicated /dev tmpfs.

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[gemini-code-assist]

Code Review

This pull request addresses an issue where libcrun failed to set up a terminal on a read-only root filesystem when /dev/console could not be unlinked. The changes introduce a fallback mechanism in libcrun_set_terminal to bind mount the PTY to /dev/console if an EROFS error occurs during the unlink operation. A new test case, test_terminal_readonly_rootfs_no_dev_tmpfs, has been added to verify this behavior by simulating a container with a read-only rootfs and no /dev tmpfs. There is no feedback to provide as no review comments were made.

[giuseppe]

LGTM