Support dynamic guest identity (UID/GID) configuration via environment variables

[doanbaotrung]

Allow users to dynamically configure the simulated guest
UID and GID at runtime via environment variables, for
example: ELFUSE_GUEST_UID and ELFUSE_GUEST_GID.

If these variables are present in the host process
environment, elfuse should parse them and override
the default guest UID/GID.

Fixes #103

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Summary by cubic

Add runtime overrides for the simulated guest UID/GID via ELFUSE_GUEST_UID and ELFUSE_GUEST_GID. The values propagate to auxv and procfs so programs see the correct identity.

• New Features
  • Parse env vars in proc_identity_init() with strict uint32 parsing; ignore invalid/empty/out‑of‑range; fall back to GUEST_UID/GUEST_GID when unset.
  • Auxv AT_UID/AT_EUID/AT_GID/AT_EGID and /proc/self/{status,task/*/status} now use proc_get_* so real/effective/saved‑set and fs IDs reflect overrides.
  • Added test-identity-override-host; built and run by make check.

^{Written for commit da2339d. Summary will update on new commits.}

[jserv]

I am not sure whether getenv is an appropriate mechanism for specifying the desired UID/GID. Instead, I am considering adopting the same approach as fakeroot.

I defer to @Max042004 for confirmation.

[Max042004]

fakeroot does two things at once:

1. Process identity — it intercepts getuid()/geteuid() so the program
   believes it is root.
2. File ownership — it intercepts chown()/stat() so ownership-sensitive
   operations stay self-consistent without real privilege.

We already implement (2). src/syscall/chown-overlay.c is literally headed
"Virtual chown overlay (fakeroot-style)":

So I prefer reframing it as the identity half of the existing overlay:
switch the interface to a fakeroot mode defaulting to root, and complete
capget/getgroups for the emulated-root case.

• Interface. fakeroot's model is "run this and you are root" — a mode, not a
  free-form numeric knob. I'd rather expose a single switch (e.g. --fakeroot /
  ELFUSE_FAKEROOT=1) that sets uid=euid=suid=0, gid=0 in one go, aligned with
  the chown-overlay.c naming, instead of asking users to hand-set two arbitrary
  integers.
• Consistency. To truly behave like root the whole identity surface has to
  agree. One real gap today: sc_capget() (syscall.c:1293) returns an empty
  capability set regardless of uid, so a tool that reads /proc/self/status
CapEff or checks a specific capability still sees "not root". A named fakeroot
  mode makes it natural to flip capget (full caps) and getgroups (gid 0)
  together; a raw UID env var leaves those half-done.