Renamed srtp_octet_string_is_eq, equal is true

[paulej]

This PR renames srtp_octet_string_is_eq() to srtp_octet_string_equal() and switches the logic of the return value. That function returned false if equal, which was opposite of what the name suggests.

However, every test is actually looking for inequality. As such, perhaps a better name might be srtp_octet_string_unequal() or something? I don't care what we name this, but I'd prefer for the name reflect what it's doing.