Fix unchecked CreateEvent calls and misc fixes

Fix unchecked CreateEvent calls and misc fixes

1)
Added missing checks for CreateEvent which also required the
following related changes:

- changed freerdp_context_new API to BOOL
- changed freerdp_peer_context_new API to BOOL
- changed pRdpClientNew callback to BOOL
- changed pContextNew callback to BOOL
- changed psPeerAccepted callback to BOOL
- changed psPeerContextNew callback to BOOL

2)
Fixed lots of missing alloc and error checks in the
changed code's neighbourhood.

3)
Check freerdp_client_codecs_prepare result to avoid segfaults
caused by using non-initialized codecs.

4)
Fixed deadlocks in x11 caused by missing xf_unlock_x11() calls
in some error handlers

5)
Some fixes in thread pool:
- DEFAULT_POOL assignment did not match TP_POOL definition
- don't free the pool pointer if it points to the static DEFAULT_POOL
- added error handling and cleanup in InitializeThreadpool