- fix assorted (harmless) off-by-one errors
- we were inconsistent on whether out->pos stays <= out->size on
  overflow; now it does, and printbuf.overflow exists to indicate if a
  printbuf has overflowed
- factor out printbuf_advance_pos()
- printbuf_nul_terminate_reserved(); use this to reduce the number of
  printbuf_make_room() calls
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>