Fix mkfifo symbolic mode handling for POSIX compliance

[Copilot]

mkfifo was incorrectly handling symbolic modes: -m +x produced 0755 instead of 0777, and -m -w failed to parse. Per POSIX, symbolic modes in mkfifo must start from a=rw (0666) and explicit -m should bypass umask.

Changes

• tree/mkfifo.rs: Added allow_hyphen_values = true to clap parser; temporarily set umask to 0 when creating FIFO with explicit -m flag
• plib/src/modestr.rs: Fixed Remove operation umask logic - changed from user &= !(rwx << 6) & !umask to user &= !(rwx << 6) | (umask & S_IRWXU) to correctly protect masked bits from removal
• tree/tests/mkfifo/mod.rs: Added 8 integration tests covering symbolic/absolute modes and error cases

Behavior

# Before: 0755 (wrong), After: 0777 (correct)
$ umask 0022
$ mkfifo -m +x fifo
$ ls -l fifo
prwxrwxrwx  # now matches GNU mkfifo

# Before: parse error, After: 0466 (correct)
$ mkfifo -m -w fifo
$ ls -l fifo
pr--rw-rw-  # umask 0022 protects group/other write from removal

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• pubs.opengroup.org
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>mkfifo wrong behavior is possible</issue_title>
<issue_description>Hi.
from posix spec

In the symbolic_mode strings, the op characters '+' and '-' shall be interpreted relative to an assumed initial mode of a=rw.

mkfifo -m +x file
prwxr-xr-x

but shall be
prwxrwxrwx

mkfifo -m -w file
error: unexpected argument '-w' found</issue_description>

<agent_instructions>Review POSIX spec for mkfifo then proceed</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes mkfifo wrong behavior is possible #550

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