* src/minikafs.c(minikafs_5settoken2): add, to set a v5 cred.
* src/minikafs.c(minikafs_5log,minikafs_5log_with_principal): take a
flag to signal that we should try rxk5, and use it and the 2b flag to
designate which method we want to use (no longer trying more than one
during a single invocation).
* src/minikafs.c(encode_bytes,encode_ubytes): take const input.
* src/minikafs.c(encode_string): add.