bintrans(1): qp switch to getopt_long

[bapt]

Summary

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

[bapt]

update usage()

[pstef]

When I imported mmencode, its usage was already established to be mmencode [-u] [-q] [-b] [-p] [-o outputfile] [file name] and has been for probably decades.
I have no opinion on what it should be, just giving historical context here.