It doesn't really have to be in ssh.c sharing that file's internal
data structures; it's as much an independent object implementation as
any of the less trivial Channel instances. So it's another thing we
can get out of that too-large source file.
It's really just a concatenator for a pair of linked lists, but
unhelpfully restricted in which of the lists it replaces with the
output. Better to have a three-argument function that puts the output
wherever you like, whether it overlaps either or neither one of the
inputs.
These are essentially data-structure maintenance, and it seems silly
to have them be part of the same file that manages the topmost
structure of the SSH connection.
