PR12592: Allow -r /path/to/kernel for stap-servers

PR12592: Allow -r /path/to/kernel for stap-servers

Previously, stap-serverd would parse the release string from a kernel
path, but now it also passes that full path on for stap to use.

* stap-serverd (process_r): Keep the full -r path for stap_options.
* stap-server (process_r): Ditto.
* session.cxx (systemtap_session::setup_kernel_release): Skip repeated
  kernel strings, for the case when a server -r /path/to/kernel is
  followed by a client's matching -r kernel.