Ruby many fixes n update to 2.1.3

Ruby was generating too big ipkg files (not nice with small storages). It seems that the
previous maintainer where moving parts of ruby-core into subpackages but did not finished the task.
Also, some subpackages simply left files in ruby-core.
Some libraries internal dependencies where optional, but not treated like that in every place
(this is in ruby upstream code).
The result of all this is the need of many mutual dependencies, that is not supported by OpenWRT, and
too much storage space, even for simple scripts. Besides that, many package simple dependencies were
missing.

In order to solve all those:

Ruby binary at (/usr/bin/ruby) was replaced by a wrap script that solves the optional dependency
of rubygems with a cmdline option. The same optional dependency of rubygems was solved in
Rdoc and minitest using patches (upstreamed).

Ruby gems that are inside Standard Library and where still in ruby-core where moved to subpackages.

ruby-core name conflicted with "Ruby Core API" semantic, which are classes aready built-in libruby/ruby
and not provided by extra files. ruby-core, in openwrt case, where just a big subset of "Standard Library API".
In order to reduce the number of subpackages that requires ruby-core (and vice versa), it was exploded into
small packages (25), including a ruby-misc that contains a much smaller subset of old ruby-core with
miscellaneous files. A new package ruby-stdlib (meta package) where added with no files requiring all ruby-* packages.

Also, ruby was updated to latest version: 2.1.3

And smaller changes:
* Doc generation was disabled in order to improve compile time (no change in output pkgs).
* fixed in ext/digest library (upstreamed)
* spin-off ruby-digest from ruby-openssl
* spin-off ruby-enc-extra from ruby-enc
* ruby-ncurses was dropped (as it was upstream)
* dropped syck references (as it was upstream)
* ruby-unit rename to ruby-testunit
* added help text for every package

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Nope, "exec" will replace the current script with the command called, so it does not continue to run after that statement.