PR16716 partial fix: Fix types in 'syscall.sched_{{get,set}param,get_priority_{max...

PR16716 partial fix: Fix types in 'syscall.sched_{{get,set}param,get_priority_{max,min},yield}'

* tapset/linux/aux_syscalls.stp: Fix handling negative args in _sched_policy_str()
* tapset/linux/nd_syscalls2.stp: Fix argstr in sched_get_priority_{max,min}
* tapset/linux/syscalls2.stp: Fix types and argstr in sched_get_priority_{max,min}
  and sched_{get,set}param
* testsuite/systemtap.syscall/mknod.c: Use thin wrapper around the syscall()
* testsuite/systemtap.syscall/sched.c: New testcase