stapdyn: add stapdyn.8 man page
Fixed readability issues in mutex-contention.stp example. No actual code change.
Fix PR15659 by updating the mount test for the nd_syscall/syscall tests.

* testsuite/systemtap.syscall/mount.c (main): Expect EPERM or ENOENT from
  failed umount() calls.
stapsh.8: new man page