Updated CreateTemporaryFile to use PNRG and match Go 1.16.
Updated CreateTemporaryFile to use PNRG and match Go 1.16.

This commit updates CreateTemporaryFile to use a pseudorandom number
generator to generate filenames on POSIX systems (it already did on
Windows since it used the Go implementation). It also brings the POSIX
implementation closer to the Go standard library implementation (at
least structurally speaking).

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen