join: -a or -v

[mknos]

• The meanings of options -a and -v are incompatible
• Usage string from standards document indicates -a OR -v [1]; this version prints a compatible usage string
• Follow OpenBSD version and fail early if both -a and -v are given

1. https://pubs.opengroup.org/onlinepubs/009604599/utilities/join.html

[briandfoy]

changes: don't allow both -a and -v in the some invocation