REJOIN processing is coded fairly opaquely; remove gotos

[terrillmoore]

See also #165. REJOIN is useful for ADR. But the logic for processJoinAccept() is somewhat diffusely placed in the code, making it hard to see what's going on. Refactor so that processJoinAccept() is called in each slot, rather than having some code that is confusingly implicitly slot dependent.

processJoinAccept() is hard to maintain; it uses chained gotos that make the flow hard to perceive. While correcting this bug, change the routine to use sub-functions instead. It will add no noticeable overhead and it will make it much easier to maintain the code.

[terrillmoore]

Found while working on #215, and bug fix will be in that branch.

[terrillmoore]

Change pushed in 86bec49 was tested in regression testing. Now that I understand how REJOIN is supposed to work, I don't think that it's wrong. May not meet the LoRaWAN spec, but... it meets the design. It now correctly triggers a on-time attempt to rejoin, which may or may not cause problems if the network sees it.