reduce iridium costs: make sure packets are "just under multiple of 50 bytes"

[jerabaul29]

It looks like the iridium provider bills 1 credit for every fraction of 50 bytes that has got started: so 49 bytes costs 1 credit, 51 bytes costs 2 credits (not sure exactly 50 bytes). Should try to be "just under" rather than "just over" to be cost effective. I thought internally fractions of credits are used, if this is actually not the case, there may be iridium cost savings to get.