bug#36282: shepherd appears to delete log-file instead of appending

[Robert Vollmert]

This is from reading the shepherd code, not verified by test currently.
Apologies if I’m missing something.

The documentation claims that log-file is appended to:

When @var{log-file} is true, it names the file to which the service's
standard output and standard error are redirected.  @var{log-file} is
created if it does not exist, otherwise it is appended to.

However, in modules/shepherd/service.scm:

   889  (define make-forkexec-constructor
[…]
   923          (lambda args
   924            (define (clean-up file)
   925              (when file
   926                (catch 'system-error
   927                  (lambda ()
   928                    (delete-file file))
   929                  (lambda args
   930                    (unless (= ENOENT (system-error-errno args))
   931                      (apply throw args))))))
   932  
   933            (clean-up pid-file)
   934            (clean-up log-file)
   935  
   936            (let ((pid (fork+exec-command command