Reduce duplicated code

[henrybear327]

Removes six instances of duplicated logic across the codebase,
folding each into a single shared definition. No behavior changes except where a
duplicate copy had a latent bug (see the FUSE write fix below).

Motivation

Several small utilities — a hex-nibble decoder, a counted path-component copy,
fd-flag manipulation, 2 MiB alignment, and a write_all loop — had been
reimplemented independently in two or more files. Each copy is a place a bug can
hide or the two can drift apart (one already had). Consolidating them gives every
call site one definition to read and maintain.

Changes

Each commit is a self-contained consolidation:

• Fail materialized FUSE writes on zero progress — fuse_write_all looped
  while (len > 0) and, on a write() returning 0, advanced by nothing and spun
  forever instead of erroring. Materializing a FUSE file for exec could hang the
  vCPU thread. Replaced with the shared write_all, which treats an unexpected
  zero-byte write as EIO.
• Drop sidecar_write_all for the shared write_all — removed a second copy
  of utils.h's write_all with identical short-write / EINTR handling.
• Use the ALIGN_2MIB macros in the Rosetta loader — align2m_up /
  align2m_down duplicated ALIGN_2MIB_UP / ALIGN_2MIB_DOWN, and two more
  sites open-coded the & ~(BLOCK_2MIB - 1) mask. Now 2 MiB rounding has one
  definition.
• Use the fd flag helpers instead of open-coding fcntl — the FIONBIO
  handler open-coded an F_GETFL / modify / F_SETFL sequence, and two
  pty-keepalive paths open-coded F_GETFD / F_SETFD | FD_CLOEXEC. Replaced
  with the existing fd_update_status_flag and fd_set_cloexec.
• Add path_component_copy for counted path components — five component
  walks in path.c and sidecar.c open-coded the same NAME_MAX+1 buffer /
  ENAMETOOLONG check / memcpy + NUL-terminate block (with the bound check
  spelled two different ways). Folded into a path_component_copy helper next to
  path_next_component.
• Share hex_nibble as the companion to bytes_to_hex — gdbstub-rsp.c
  (hex_val) and sidecar.c (hex_nibble) each carried the same hex-digit
  decode ladder, the inverse of the existing bytes_to_hex encoder. Moved to
  utils.h so both directions of the codec live together.

---

Summary by cubic

Consolidates six duplicated helpers into shared utilities, reducing drift and simplifying maintenance. Also fixes a vCPU hang by failing FUSE materialization on zero‑progress writes; no other behavior changes.

• Bug Fixes

  • Materialized FUSE writes now error on zero‑byte progress (EIO) instead of spinning forever.

• Refactors

  • Use ALIGN_2MIB_UP/ALIGN_2MIB_DOWN in Rosetta; remove open‑coded masks.
  • Replace open‑coded fcntl sequences with fd_set_cloexec and fd_update_status_flag.
  • Add path_component_copy and use it for counted components in path.c and sidecar.c.
  • Move hex_nibble to utils.h; reuse in gdbstub-rsp and sidecar.
  • Remove sidecar_write_all and call shared write_all.

^{Written for commit 76d9378. Summary will update on new commits.}