devpts: return ESPIPE for positional writes to PTYs

[ryux1]

Description

PTY master and replica file descriptions currently fall through to the default
PWrite implementation, which returns EINVAL. Mark both endpoints as denying
positional writes so VFS returns ESPIPE before dispatching the operation.

This matches Linux for pwritev and pwritev2 with explicit offsets while
preserving pwritev2 with offset -1 as streaming I/O. It also lets callers
such as Zig's file writer recognize PTYs as unseekable and fall back to normal
streaming writes.

Fixes #13737.

Tests

• //test/syscalls:pwritev2_test_runsc_systrap_directfs
• //test/syscalls:pwritev2_test_runsc_ptrace
• //test/syscalls:pwritev2_test_native (focused regression)
• //pkg/sentry/fsimpl/devpts:devpts_test
• //pkg/sentry/fsimpl/devpts:devpts_nogo
• //pkg/sentry/fsimpl/devpts:devpts_test_nogo

Assisted-by: Codex

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