Remove file-wide unsafe libc calls pragma from c/b/webauthn

-- Re-write expressions using more modern equivalents.
-- Mark remaining expressions as UNSAFE_TODO

Change-Id: If8135220a0193a8496213f3e7de1c9ba698880d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6471349
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448659}