Refactor process identity to support unified fakeroot mode

[doanbaotrung]

Refactor process identity to support unified fakeroot mode

Replace the raw numeric UID/GID overrides (ELFUSE_GUEST_UID and
ELFUSE_GUEST_GID) with a unified --fakeroot CLI switch and
ELFUSE_FAKEROOT=1 environment variable.

This change aligns the process identity surface with the existing
fakeroot-style chown-overlay. When fakeroot mode is enabled:

• The emulated process UID, EUID, SUID, GID, EGID, and SGID are set
  to 0 (root).
• sc_capget() returns full capability sets, with the high capability
  word masked to LINUX_CAP_LAST_CAP to prevent impossible capabilities.
• sys_getgroups() returns a single supplementary group (GID 0).

Fakeroot mode defaults to false (disabled). In this default state, the
guest process runs with an unprivileged emulated identity (UID/GID 1000).

Limitations of fakeroot mode:

• It only emulates root status on the guest-identity surface (getuid,
  geteuid, capget, getgroups, etc.) and virtual file attributes inside
  the guest (chown-overlay).
• It does NOT grant real host-level root privileges, nor does it bypass
  macOS sandboxing, file system permissions, or other host-level security
  constraints.

Also increase child_argv size in forkipc.c to 16 to prevent overflow
when propagating optional flags, and update host and guest tests to
adapt to the fakeroot behavior.

---

Summary by cubic

Introduce a unified fakeroot mode that emulates root identity and capabilities, replacing numeric UID/GID overrides. This lets package managers and other root-checking tools run without host sudo.

• New Features

  • --fakeroot flag and ELFUSE_FAKEROOT=1 env enable root emulation (default off).
  • When enabled: all UIDs/GIDs are 0; capget reports full sets (masked to LINUX_CAP_LAST_CAP); getgroups returns [0].
  • Fork/clone children inherit --fakeroot.

• Refactors

  • Removed ELFUSE_GUEST_UID/ELFUSE_GUEST_GID; AUX vector and /proc/*/status now read from proc_get_*.
  • Increased fork child argv capacity to 16 to safely propagate optional flags.
  • Tests updated; added test-identity-override-host and hooked into make check.

^{Written for commit 4a5626e. Summary will update on new commits.}

[doanbaotrung]

Fix #103

[jserv]

Remove this blank line.

[doanbaotrung]

fixed

[jserv]

Mention the default value (false) of fakeroot mode and its limitations in git commit messages.

[doanbaotrung]

Updated