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
This change is based on the "Principles of Secure Coding" course by
Matthew Bishop, PhD., which specifically discusses this code in FreeBSD.
(This change redoes r368776 due to a silly mistake)