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/reader.h

@@ -227,9 +227,6 @@ wcstring completion_apply_to_command_line(const wcstring &val_str, complete_flag
                                           const wcstring &command_line, size_t *inout_cursor_pos,
                                           bool append_only);
 
-/// Terminate all background jobs
-void kill_background_jobs();
-
 /// Print warning with list of backgrounded jobs
 void reader_bg_job_warning();