Resolve "Errno 115 returned on successful completion for several system calls"

Closes #78 (closed)

This fix contains three parts:

1. errno 115 was leaked during initialization of the client in init_ld_environment_(). However, the value on its own is irrelevant to this issue as all internal errno variable changes should be kept internal as it is the case during each syscall intercept hook. Therefore, the original errno variable is restored after the initialization finishes.
2. lseek() tests revealed that whence SEEK_SET was not handled correctly. It should not allow negative offsets.
3. All FIXME TODOs in the testing code have been removed including testing on the errno in a successful case because it is undefined.