seq: detect and report I/O errors immediately

seq: detect and report I/O errors immediately

Ensure I/O errors are detected (and terminate seq), preventing seq
from infloop (or running for long time with a large
range) upon write errors or ignored SIGPIPE. Examples:

     seq 1 inf > /dev/full             (seq_fast)
     seq 1.1 0.1 inf >/dev/full        (print_numbers)

* src/seq.c (io_error): A new function to diagnose appropriate
stdio errors and exit the program with failure status.
(seq_fast, print_numbers): Explicitly check for write errors
and terminate the program with diagnostic.
* tests/misc/seq-io-errors.sh: Test error detection with /dev/full.
* tests/misc/seq-epipe.sh: Test error detection with broken pipes.
* tests/local.mk: Add new tests.
* NEWS: Mention the fix.