Under Unix, create lockfile in ~, not /tmp.

This fixes a potential denial-of-service attack,
where another user can prevent you running Plover
by creating a file in /tmp with your name in the
filename.

This patch was requested by Paul Wise in
<http://bugs.debian.org/710989> as part of the
preparations for including Plover in Debian.
Merge pull request #88 from tthurman/master

Create lockfile in ~