core: Remove POSIX threads influence from synchronization code

core: Remove POSIX threads influence from synchronization code

This commit changes the signatures of the synchronization functions
to reflect the needs of the library rather than the signature of the
pthreads API. The mutex and condition variable attributes parameters
have been removed as no part of the core library makes use of them.

In addition, the condition variable timed-wait function has been modified
to accept the relative time passed in via libusb_wait_for_event(). This
allows the implementation-specific code to handle conversion to absolute
time as necessary, rather than forcing this to occur.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>