print access error through fio_perror

Interface fio requires romfs to implement a reference of fio_err, and it
must store a string of the last error message. It lets error messages be
arranged by romfs.

Issues:
1. Should fio_perror be a critical section? So far, it reads data from
   ""-enclosed static strings, and how about an array char[] is given?
2. Any idea of implementing it with errno.h or other standard libraries?