On client shutdown write directly into "peers.dat"...

...and not into a temporary file which gets renamed to "peers.dat" later.
This prevents softlinks from being replaced by a "real" file, see http://jira.darkcoin.qa/browse/DRK-122

Change was already merged (see https://github.com/darkcoin/darkcoin/pull/57), but got somehow lost because of a forced update by Evan.

Please merge again.