rpc2: RPCSEC_GSS privacy service (krb5p)

[benjarvis]

Summary

Adds the RPCSEC_GSS privacy service (krb5p) to the rpc2 GSS layer, completing
the set alongside the krb5/krb5i support merged in #108.

• Call path: arguments arrive wrapped as rpc_gss_priv_data { opaque databody_priv<> }, where databody_priv is the GSS-sealed token of
  XDR(seq_num) || XDR(proc_args). evpl_rpc2_gss_unwrap_privacy calls the
  provider's unwrap (gss_unwrap), verifies the embedded seq against the
  credential, and repoints the inner proc arguments for normal dispatch.
• Reply path: evpl_rpc2_gss_wrap_reply_privacy GSS-wraps
  seq_num || results and emits it as the single opaque of rpc_gss_priv_data.
• EVPL_RPC2_GSS_SVC_PRIVACY is now accepted in the DATA path instead of being
  rejected with AUTH_TOOWEAK.

The crypto stays entirely behind the existing provider vtable (wrap/unwrap),
so libevpl remains Kerberos-free.

Latent fix: GSS unwrap scratch arena overflow

While testing krb5p against the real Linux kernel client, a 256 KiB WRITE
failed to unwrap. Root cause: both the privacy databody_priv token and the
(pre-existing) integrity databody were gathered into the request's fixed
128 KiB dbuf arena (xdr_dbuf_alloc_space), so any wrapped call larger than
the arena returned NULL and failed. This fix mallocs the gather scratch
(bounded by the message size, not the arena) and frees it. The inner iovec
already uses evpl_iovec_alloc, and the reply-wrap paths already use
evpl_iovec_alloc/malloc, so large encrypted READ replies were never
affected — and large krb5i writes are fixed by the same change.

Testing

Validated end-to-end in chimera (the consumer): pynfs 4.1 sec=krb5p smoke +
per-export sec=krb5p policy, and the real Linux kernel client mounting
-o sec=krb5p across four guest kernels (Ubuntu 22.04/24.04/26.04) driving a
multi-block encrypted WRITE+READ. With the arena fix the unwrap failures (and
the client's context-reestablishment retries they triggered) are gone; krb5,
krb5i and krb5p all pass clean. rpc2's own transport ctests pass.