#19 fix race condition in open(), making computer freeze

[cristi]

This should fix https://sourceforge.net/tracker/index.php?func=detail&aid=3187007&group_id=178178&atid=884191

There is a race condition in the opening function - mutex_lock should be called before the check for device being already opened.
Attach patch - made to tag 2.1.0 - but it should be applied to trunk as well. Also, the test program.
(note - due do my distro + another patch to the source - I manually edited the patch file. Hope it can still be applied automatically, by 'patch')

[cristi]

patch to fix device open() race conditions and computer freeze

[cristi]

program to compile and run to replicate problem