fix failure of pthread_setschedparam to pass correct param to kernel

the address of the pointer, rather than the pointer, was being passed.
this was probably a copy-and-paste error from corresponding get code.

add support for thread scheduling (POSIX TPS option)

linux's sched_* syscalls actually implement the TPS (thread
scheduling) functionality, not the PS (process scheduling)
functionality which the sched_* functions are supposed to have.
omitting support for the PS option (and having the sched_* interfaces
fail with ENOSYS rather than omitting them, since some broken software
assumes they exist) seems to be the only conforming way to do this on
linux.