bucoord: Use mkstemp properly

bucoord: Use mkstemp properly

Use mkstemp wherever we have it available, to silence warnings
about mktemp's safety.

When we do use mkstemp, use it properly. It doesn't return void,
it returns an open filehandle. Convert this filehandle into a FILE *,
rather than throwing it away, and leaking an open file descriptor.

Caught by clang-analyzer

Change-Id: Ib5864f684c6d5a8686e0a872364a645c90a29390
Reviewed-on: http://gerrit.openafs.org/2144
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>