[openbsd-compat/fmt_scaled.c openbsd-compat/openbsd-compat.h]
Fix compilation on Linux, including pulling in fmt_scaled(3)
implementation from OpenBSD's libutil.
[clientloop.c clientloop.h ssh.c mux.c]
tidy up session multiplexing code, moving it into its own file and
making the function names more consistent - making ssh.c and
clientloop.c a fair bit more readable.
ok markus@