don't crash if ibrowse is already started

fix #COUCHDB-1986
Improve docs of use_checkpoints and checkpoint_interval config options.
Fix make distcheck
Ship single full document default config file.

The default.ini becomes really solid default config file with full set of
options with short documentation and default values. The big warning alerts you,
that any changes made in default.ini might be overwritten and suggests you
to use local.ini for any modifications instead.

The local.ini is removed and doesn't ships with new CouchDB installations.
This also resolves issue when after CouchDB upgrade package manager asks users
to merge their local.ini with shipped one. However, when CouchDB starts it
checks for local.ini existence and will create it if missed to keep there all
config options modifications.

COUCHDB-1744