Use stat(2) to retrieve the file permissions mode

Use stat(2) to retrieve the file permissions mode

Remove the mode arguments from util_file_prepend_line(),
util_file_pop_line() and util_file_set_content().

Both util_file_prepend_line() and util_file_pop_line() just calls
util_file_set_content() so stat(2) can be used there and if it
fails the 0600 is used as a fallback.

Thanks to @fanglingsu for reviews and suggestions!