Fix the read/write auto-transition bugs. Note that if we can't

Fix the read/write auto-transition bugs.  Note that if we can't
auto-transition, we fail the operation.  This is different than glibc's
apparent behavior for writing of clearing the read buffer and still
failing the write without setting the stream's error flag.

Also, change a number of "errno = xxx" assignments to use __set_errno().

Also, change setvbuf(file, NULL, _IO{LF}BF, 0) behavior to more closely
match glibc's by keeping the current buffer and only changing the buffering
mode.  Update setlinebuf() in the process to match the man page behavior.