PR16716 partial fix: Fix types in 'syscall.{pread,pwrite,read,write}'.

PR16716 partial fix: Fix types in 'syscall.{pread,pwrite,read,write}'.

* tapset/linux/syscalls2.stp: Fixed types in syscall.pread,
  syscall.pwrite, syscall.pwrite32, syscall.read, and syscall.write.
* tapset/linux/nd_syscalls2.stp: Ditto.
* testsuite/systemtap.syscall/readwrite.c (main): Added tests and split.
* testsuite/systemtap.syscall/lseek.c: Tests moved from readwrite.c and
  new tests added.
* testsuite/systemtap.syscall/pread.c: Ditto.
* testsuite/systemtap.syscall/pwrite.c: Ditto.