Fix -C disabling checking rather than checking quietly.

Fix sort(1) exiting 1 on certain errors, as POSIX requires sort(1) to only
exit if the input wasn't sorted when -c.

Fix -o opening the output file for truncation before all the input has been
read, as POSIX requires allowing -o to be an input file.

POSIX requires sort(1) to handle input errors by either erroring with no
output, or by erroring and sorting the input read so far. Change the current
behavior of continuing to the next file to simply failing hard on the first
input error.

Don't increment the last line number on the end of the standard input.

Report -c/-C as incompatible with -o.

Exit unsuccessfully on any output errors.

Update to current coding conventions and add documentation while here.