Now gmid doesn't touch anymore the internals of the imsg structs.

for some reason that's not entirely clear to me, __dead doesn't seem to work on macos, so clang thinks datalen is used un-initialized. meh

soon we'll be using a struct ibuf and it'll be confusing.

Makes parsing and handling of imsgs simpler / clearer. only crypto.c is left as-is.

since proc_forward_imsg() never forwards a file descriptor (it's never called actually) just use -1 there.