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
This avoids interference caused when two sockets are bound to the
same port in different multicast groups.
Fixes#241
Change-Id: Iba1b3300e3850f1f268886b7a49f5a3315d95b80