fix(tty): preserve OPOST chars when output is full

[fslongjin]

Summary

• distinguish OPOST output-buffer-full from valid zero-output consumption in n_tty
• keep ONLCR-expanded characters pending when PTY output space cannot fit the full expansion
• provide /bin/true and /bin/false from coreutils in the rootfs for Linux-compatible absolute paths used by gVisor tests

Root Cause

process_output_char_to_buf() returned ENOBUFS with an empty temporary output buffer when OPOST | ONLCR needed to expand \n into \r\n but only one output byte of room was available. The caller then treated the empty buffer as a valid consumed-without-output character and advanced the input offset, silently dropping the input newline.

Linux n_tty keeps these states distinct: output full means retry without consuming input, while ONOCR at column zero is a valid zero-output consume.

The follow-up Integration Test failure in Waiters/WaitSpecificChildTest.AfterChildExecve/{0,1} matched the current origin/master baseline failure at e7331811004d725d8303b2ee01878d425ebba63d: the test execs /bin/true, while the DragonOS rootfs only installed coreutils true under /usr/local/bin/true. The child therefore exited with ENOENT (2), and wait observed status 512 / 2.

Validation

• make kernel
• make user
• TOML parse check for user/dadk/config/all/core_utils-9.4.0.toml
• Verified generated coreutils build output and local sysroot contain static ELF files at:
  • bin/dadk_cache/build/core_utils_9_4_0/bin/true
  • bin/dadk_cache/build/core_utils_9_4_0/bin/false
  • bin/sysroot/bin/true
  • bin/sysroot/bin/false
• Earlier PTY-focused validation:
  • make fmt
  • FORCE_UNSAFE_CONFIGURE=1 DUNITEST_PATTERN=normal/tty_pty_hangup make test-dunit
    • The current Makefile did not propagate DUNITEST_PATTERN into the guest, so this ran full dunitest.
    • Result: 419 total, 410 passed, 0 failed, 9 skipped.
    • TtyPtyHangup.TciflushDoesNotDiscardLargeOpostSlaveOutput passed in 27 ms.

[fslongjin]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: 45a3f3f3e4

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