Add recursive mutexes to threading abstraction

Add recursive mutexes to threading abstraction

This is necessary for the device close path which needs to attain the
events lock, but which might itself be called while handling an event.
The events lock is necessary to properly clean up transfers which might
still be pointing to the device. References #82.

[stuge: Move usbi_mutex_init_recursive() into threads_posix.c]
[stuge: Must also #define _XOPEN_SOURCE 500 to be able to build]
[pbatard: Un-inline usbi_mutex_init_recursive() to make Cygwin happy]