ImportantFileWriter::WriteFileAtomically uses mkstemp internally,
which set file permission to 0600, which is not what we want.
Update the code to not use mkstemp instead.
Also updated temporary file name logic to include process id in
the name so it can be unique across processes.
Fixes#259
Change-Id: I2d5a375925cf552bc0db5269f409d7522e63fca5