sort(1): Fix -m when only implicit stdin is used for input

sort(1): Fix -m when only implicit stdin is used for input

Observe:

printf "a\nb\nc\n" > /tmp/foo
# Next command results in no output
cat /tmp/foo | sort -m
# Next command results in proper output
cat /tmp/foo | sort -m -
# Also works:
sort -m /tmp/foo

Some const'ification was done to simplify the actual solution of adding "-"
explicitly to the file list if we didn't have any file arguments left over.

PR:		190099
MFC after:	1 week