bintrans(1): qp switch to getopt_long

In preparation for more arguments, switch bintrans qp argument parsing
to getopt_long, while here make the decodign argument being -d|--decode
for compatibility with base64 encoding/decoding

MFC After: 1 week
Reviewed by: pstef
Differential Revision: https://reviews.freebsd.org/D48380

(cherry picked from commit a8d9bd3fa5855fe7583ed05946296ab6b9937d69)