Merge branch 'jk/write-file'

General code clean-up around a helper function to write a single-liner to a file. * jk/write-file: branch: use write_file_buf instead of write_file use write_file_buf where applicable write_file: add format attribute write_file: add pointer+len variant write_file: use xopen write_file: drop "gently" form branch: use non-gentle write_file for branch description am: ignore return value of write_file() config: fix bogus fd check when setting up default config