factor out macro expansion from sendbuf() function
cleanup

- refactor all declarations of 'extern char buffer[]' to sendbuf.h
  preparing a later drop of sendbuf() function and replacement by
  sendbuf(char *msg).
- Add include guards and sort out #include declarations.
Use sendmessage(msg) instead of sendbuf()
merge speedup.c and speeddown.c to speedupndown.c

- there is a lot of duplicated code which can be factored out
Refactor speedup() and speeddown()
use sendmessage() instead of sendbuf()
cleanup keyer() - first part

- use sendmessage() instead of sendbuf()
- cleanup some unused code
cleanup keyer() - second part

- correct logic for sending - do not send control characters to the
  keyer
- drop no longer used logic to buffer input and display it via buffer[]
  and buloc. That code was not functional for a long time. Even in
  0.9.10 it was no longer used correctly.
make sendbuf() and buffer[] private

- new code should use sendmessage() instead