Site tuning defaults: Improve documentation; internal improvements

Site tuning defaults: Improve documentation; internal improvements

Two documentation fixes:

* Quote the correct default value for setup-timeout
  (setup_retry_interval) in the README.
* Improve the documentation of the default value for renegotiate-time.

Three simple internal improvements:

* Change definitions of DEFAULT_* to forms which make the semantics
  of the values clearer, thus obviating the need for the comments
  giving human-readable values.
* Add comments to DEFAULT_* time values giving the units (currently,
  they are all in millisecons).
* Rename (in the code) SETUP_TIMEOUT and setup_timeout to
  SETUP_RETRY_INTERVAL, which is more accurate.  We leave the config
  dictionary entry with the misleading name (particularly, since we
  have no facility for spotting obsolete or misspelled config keys).

Also two changes which are preparatory to the mobile peers support:

* New macro DEFAULT(D) which currently simply expands to DEFAULT_*.
* New convenience macro CFG_NUMBER for a common pattern of use for
  dict_read_number, which uses DEFAULT().

No intentional functional changes.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>