fix erroneous return of partial username matches by getspnam[_r]

fix erroneous return of partial username matches by getspnam[_r]

when using /etc/shadow (rather than tcb) as its backend, getspnam_r
matched any username starting with the caller-provided string rather
than requiring an exact match. in practice this seems to have affected
only systems where one valid username is a prefix for another valid
username, and where the longer username appears first in the shadow
file.