libc/resolv: get rid of MD5

MD5 is used by libc/resolv to generate a random sequence id from a
current time stamp. Replace this convoluted mechanism with a call
to arc4random(). This permits us to entirely drop MD5 from libc,
simplifying the MD5 rework proposed in D45670.

Approved by: markj
Reviewed by: kevans, markj
See also: D45670
Event: EuroBSDcon 2025
Differential Revision: https://reviews.freebsd.org/D52784