non-seekable files like FIFOs can be used for logs other
than the binlog. (Bug #8271)


include/my_sys.h:
  Add APPEND_CACHE cache type
mysys/mf_iocache.c:
  Handle APPEND_CACHE cache type to avoid calling my_seek() and my_tell()
  on logs opened O_APPEND that we never read from (so they can be a
  non-seekable file like a FIFO).
sql/mysqld.cc:
  Use APPEND_CACHE for log files not set to SEQ_READ_APPEND.