include_server --no_force_dirs inhibits forcing_technique

The "forcing_technique" trick which handles the (rare) case of files
included as e.g. <foo/../file.h> causes a performance degradation:
The forcing files accumulate over the life of the include_server, making
distcc send more and more (useless) files as the build progresses.
While the forcing files are zero-length, the transmission filenames
themselves introduce a significant overhead for builds with many files
and directories.

This patch implements the include_server arg --no_force_dirs, which if
set switches off the "forcing_technique" scheme.

The HTML files were generated automatically using

  sudo apt-get install troffcvt
  make man-html

But I have also changed the man source files to ensure that
the generated output still looks nice.  The changes were
mainly replacing blank lines with ".PP", but there were a
few other tweaks needed too.

Tested by viewing the output on both Chrome and Safari,
and using "man" and (equivalently) "nroff -man" and
examining the changes.

Reviewed by Craig Silverstein.

Resolves <http://code.google.com/p/distcc/issues/detail?id=75>.

assumptions of distcc-pump added to man/distcc.1 as well. The BUGS section
of distcc.1 has been updated.

A spelling error correction is made to man/distcc.1. 

The pump.in script is changed so that it does not introduce other certain
defaults than those mentioned in the manual (which are those of the of include
server).

Finally, I changed the order of the options in the include server source code to
be alphabetical in accordance with include_server(1) man page.

Reviewers: fergus, csilvers.