sys: Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

Bring in compatibility glue for libsodium

The idea is untouched upstream sources live in sys/contrib/libsodium.

sys/crypto/libsodium are support routines or compatibility headers to allow
building unmodified upstream code.

This is not yet integrated into the build system, so no functional change.