Remove caching from getlogin(2).

This caching has existed since the CSRG import, but serves no obvious
purpose. Sure, setlogin() is called rarely, but calls to getlogin()
should also be infrequent. The required invalidation was not
implemented on aarch64, arm, mips, amd riscv so updates would never
occur if getlogin() was called before setlogin().

Reported by: Ali Mashtizadeh <ali@mashtizadeh.com>
Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14965