Wrapper (Unix): use SIGSTOP instead of SIGTSTP to suspend subprocesses

[davidpanderson]

The former can't be caught, the latter can.
The goal was to support wrapped program that are themselves wrappers of some sort.

Problem (as reported in #5840): contrary to documentation, SIGTSTP doesn't actually stop processes not connected to a TTY (at least on Linux - I checked).

This PR changes the default signal back to SIGSTOP, but provides the option (via a --use_tstp cmdline arg to wrapper) to use SIGTSTP instead.

Fixes #5840

[AenBleidd]

@davidpanderson,
mingw and windows builds are failing with the next errors (both are basically the same):

         D:\a\boinc\boinc\samples\wrapper\wrapper.cpp(1007,9): error C2660: 'suspend_or_resume_descendants': function does not take 2 arguments [D:\a\boinc\boinc\win_build\wrapper.vcxproj]
         D:\a\boinc\boinc\samples\wrapper\wrapper.cpp(1009,9): error C2660: 'suspend_or_resume_process': function does not take 3 arguments [D:\a\boinc\boinc\win_build\wrapper.vcxproj]

../samples/wrapper/wrapper.cpp:1007:54: error: too many arguments to function ‘void suspend_or_resume_descendants(bool)’
 1007 |         suspend_or_resume_descendants(false, use_tstp);
      |                                                      ^
In file included from ../samples/wrapper/wrapper.cpp:84:
../lib/proc_control.h:38:[13](https://github.com/BOINC/boinc/actions/runs/7453969303/job/20280387980?pr=5486#step:7:14): note: declared here
   38 | extern void suspend_or_resume_descendants(bool resume);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../samples/wrapper/wrapper.cpp:1009:55: error: too many arguments to function ‘void suspend_or_resume_process(int, bool)’
 1009 |         suspend_or_resume_process(pid, false, use_tstp);
      |                                                       ^
In file included from ../samples/wrapper/wrapper.cpp:84:
../lib/proc_control.h:39:13: note: declared here
   39 | extern void suspend_or_resume_process(int pid, bool resume);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~

[davidpanderson]

Oops! Fixed now

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (95adf93) 10.84% compared to head (86f6f17) 10.84%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5486   +/-   ##
=========================================
  Coverage     10.84%   10.84%           
  Complexity     1068     1068           
=========================================
  Files           279      279           
  Lines         36158    36158           
  Branches       8356     8356           
=========================================
  Hits           3920     3920           
  Misses        31844    31844           
  Partials        394      394           

Files	Coverage Δ
lib/proc_control.cpp	0.00% <0.00%> (ø)