- Changed the {read,write}_longint() functions into separate

- Changed the {read,write}_longint() functions into separate
  {read,write}_varlong() and {read,write}_longint() functions:
  the former handles variable-length transmissions, and the
  latter is backward-compatible with older rsync versions.
- Improved the {read,write}_var{int,long}() functions to be
  simpler and to use a different byte order.
- The {read_write}_varlong() functions also allow the caller
  to specify the minimum number of bytes to send.  This allows
  time values to be sent efficiently with a 4-byte minimum,
  while length values continue to be sent with a (more optimal
  for them) 3-byte minimum.