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.

hardcoded 60 second backoff period should be tunable

Makes the backoff behavior tunable via a new env var DISTCC_BACKOFF_PERIOD.
The default backoff remains set for 60 seconds.

"all servers busy" dcc_lock_pause is hardcoded at 1 sec, could be tunable.

Makes the pause time tunable via a new env var DISTCC_PAUSE_TIME_MSEC.
The default remains set for 1 second.

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.

for the --whitelist option to make it clear that distcc will accept
connections *only* from users on the list.

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

add an environment variable DISTCC_SKIP_LOCAL_RETRY for skipping
the local retry in case of a remote compilation failure.

Reviewed by Fergus Henderson.