PR16716 partial fix: Better types in 'syscall.{getpriority,setpriority}'.
* tapset/linux/syscalls.stp: Fix types in 'syscall.getpriority'.
* tapset/linux/syscalls2.stp: Fix types in 'syscall.setpriority'.
* tapset/linux/aux_syscalls.stp (_priority_which_str): Convert to use
_stp_lookup_str().
* testsuite/systemtap.syscall/getpriority.c: New test case.
* testsuite/systemtap.syscall/setpriority.c: Ditto.