Fixed PR13977 by updating the rlimit.exp testcase.

* testsuite/systemtap.base/rlimit.exp: Added 2 fixes. Add
  '--disable-cache' to the 'CPU small' subtest. Remove the 'STACK small'
  subtest since some kernels ignore the small stack limit.
Ensure there's a session for setup_dwfl_kernel

It was tripping on the assert added in commit 044a265a, when running in
any mode that didn't previously set current_session_for_find_debuginfo.
e.g. stap -e 'probe begin { exit() }' -p3 -d /usr/bin/ls
Updated testsuite/systemtap.examples/process/mutex_contention.stp example.

Stats that were supposed to be collected per FUTEX_WAIT call site were
being collected per mutex init stack. So, in the "aggregate stats" section
of the report, you'd see a bunch of stacks with pthread_mutex_init at the
top, which was very confusing because a futex isn't contended when it's
created.
PR15673: Enable deref.exp for stapdyn, and fix writes

* runtime/dyninst/copy.c (__copy_to_user): Fix the to/from pwrite args.
* runtime/loc2c-runtime.h (CATCH_DEREF_FAULT): Move here for all to use.
* testsuite/systemtap.base/deref.exp: Enable test for all runtimes.
* testsuite/systemtap.base/deref.stp: Use a known function pointer
  value, not the kernel-centric "current".
Mark externalvar.exp as KFAIL on stapdyn, PR15052