Request change of ptrace scope from KCrash

Summary:
- On Linux recent kernels only allow attaching a debugger
  to a process if
  * the right to attach to processes is enabled system-wide
  * the debugger is an ancestor of the debuggee
  * or the debuggee sets a tracer process by a call to
    `prctl(PR_SET_PTRACER, debugger_pid, 0, 0, 0);`
- DrKonqi will ask the debuggee by a socket connection to
  set a new ptracer. This is required if an external debugger
  is started (using the usually hidden Debug button).
- When the debugger has finished, the access is requested back
  to DrKonqi to allow internal backtraces again.

Test Plan:
- try to attach gdb or KDevelop to the process by
  using the Debug button: operation will not be permitted
- apply corresponding patches to KCrash D11236 and DrKonqi
- the process can now be debugged
- after detaching the debugger again, a backtrace can be
  created using the DrKonqi dialog as usual

Reviewers: #plasma_workspaces, #frameworks, ossi

Reviewed By: ossi

Subscribers: ossi, lepagevalleeemmanuel, maximilianocuria, adridg, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11235