setlogincontext(): Comply to style(9)

Remove indentation by inverting the big 'if (lc)' and using 'return'.
Use explicit binary operators to produce booleans.

Reviewed by: emaste, kib, dchagin
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40346