This can prevent bizarre failures if, for example, you've done a
configuration in the top-level source directory, leaving behind one
config.h file, and then do an out-of-tree build in another directory,
with different configuration options. This way, we always pick up the
same config.h, in the build directory.
"Text protocols" are protocols that have the general feel of FTP, with
command lines with a command name and space-separated arguments and
response lines beginning with a 3-digit reply code. They can also
include HTTP-style headers and an entity body.
We add support for the FTP control channel, HTTP, SMTP, and RTSP. We
also change the SIP printer to use it.
