Fix hup_background_jobs (née kill_background_jobs) implementation

This was introduced in 1b1bc28c0a but did
not cause any problems until the job control refactor, which caused it
to attempt to signal the calling `exec` builtin's own (invalid) pgrp
with SIGHUP.

Also improved debugging for `j->signal()` failures by printing the
signal we tried sending in case of error, rename the function to
`hup_background_jobs`, and move it from `reader.h`/`reader.cpp` to
`proc.h`/`proc.cpp`.

src/parse_execution.cpp

@@ -786,7 +786,7 @@ parse_execution_result_t parse_execution_context_t::populate_plain_process(
                 return parse_execution_errored;
             }
             else {
-                kill_background_jobs();
+                hup_background_jobs();
             }
         }
     }