login(1): when exporting variables check the result of setenv(3)

When exporting a variable we correctly check all the preconditions that
could make setenv(3) fail. Checking the setenv(3) return value seems
redundant, but given that login(1) is critical, it doesn't hurt to have
a post-check.

This change is based on the "Principles of Secure Coding" course by
Matthew Bishop, PhD., which specifically discusses this code in FreeBSD.

Differential Revision: https://reviews.freebsd.org/D26966