Make I/O errors fatal instead of mere warnings (#63)

An input/output error indicates a fatal condition, even if it
occurs when closing a file. Awk should not return success on I/O
error, but treat I/O errors as it already treats write errors.

Test case:

$ (trap '' PIPE; awk 'BEGIN { print "hi"; }'; echo "E $?" >&2) | :
awk: i/o error occurred while closing /dev/stdout
 source line number 1
E 2

The test case pipes a line into a dummy command that reads no
input, with SIGPIPE ignored so we rely on awk's own I/O checking.
No write error is detected, because the pipe is buffered; the
broken pipe is only detected as an I/O error on closing stdout.

Before this commit, "E 0" was printed (indicating status 0/success)
because an I/O error merely produced a warning. A shell script
was unable to detect the I/O error using the exit status.

0 files changed, 0 insertions, 0 deletions