base: don't overwrite errno in unique_fd::~unique_fd.

unique_fd's destructor potentially mangling errno makes it difficult to
use correctly in code that sets errno (or, in reality, it makes it so
that errno values get randomly stomped upon if close actually sets
errno, because no one accounts for this case).

Preserve errno ourselves to avoid this.

Test: treehugger
Change-Id: Ib06e6f65866d86fff4032b2311021eaf9226a1af