changes ln from third to first form (see man ln)

The stdout output of gluon-web scripts is directly sent to uhttpd,
becoming a part of the HTML output or even replacing HTTP status or
headers. The output of gluon-reconfigure is not supposed to end up
there.

While we're at it, also add an exec to avoid an unnecessary shell
process.

This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

A missing uci save for gluon-setup-mode caused the initial setup-process
to be stuck in a config-mode loop.

Simplify the individual form providers by always running reconfigure. This
also commits all UCI packages, so we can remove all tracking of UCI
sections from the wizard code.

Instead of returning a list of packages to commit, the providers just
save their settings directly. Explicit runs of upgrade scripts are
removed, as they are now run by reconfigure.

- CGI script and index.html are moved from gluon-web to
  gluon-config-mode-core, the script is renamed to 'config'
- gluon-web and gluon-web-model base views and i18n files are symlinked
  into the new path
- gluon-web-theme is renamed to gluon-config-mode-theme and installs
  directly into the new path
- all gluon-web-* models, controllers and views are moved into the new
  path

exec() is moved to gluon.util.

These files are already located in /lib/gluon, no need to add it a second
time.

Allow returning functions in addition to the names of UCI packages to
commit. Functions are run after all packages have been committed.

Some files have received some additional refactoring.

As PROVIDES can be used to replace real packages now, we don't need the
virtual packages as workaround anymore. This also means that the providing
packages don't need to be added to site.mk explicitly anymore when the
default provider is used.

Make use of our new _translate() function.

We also make use of the boolean support of simple-uci to make scripts
clearer.

Fixes #414

Allows weird release numbers, models and site names, and prepares support
for pretty hostnames.

Don't fork reboot process before all package hooks have been handled and
rendering is complete.

Replace debug.setfenv hack to close stdout with nixio.dup.

Fixes #772

The expert mode can still be useful without any cohfig mode modules.

This will require site.mk to explicitly pull in the gluon-setup-mode and
gluon-config-mode-core packages.