[PATCH 1/2] lib: Add str_equals_timing_safe()

[PATCH 1/2] lib: Add str_equals_timing_safe()

Constant-time string comparison that avoids the length leak in
str_equals_timing_almost_safe(). Compares HMAC-SHA256 digests of the
inputs (keyed with hash_iv) rather than the strings themselves, so
neither the contents nor the length of either input affects timing in
a way an attacker can exploit.

Gbp-Pq: Name 0001-lib-Add-str_equals_timing_safe.patch