Adds the common skalibs library and the execline shell-like language.

I'm wondering if the commands of the language belong in bin or if they
maybe should be in libexec/execline. They have pretty generic names
and might clash with other packages.

Either way I think patch #0003 is important, as it makes a script like:

#!/gnu/store/vragmy67fri71aamzp3ifbxf00n27qyy-execline-2.1.2.2/bin/execlineb -P
foreground { sleep 1 } echo blah

... work at all times without writing
/gnu/store/vragmy67fri71aamzp3ifbxf00n27qyy-execline-2.1.2.2/bin/foreground.
Having access to the language without having to have it in $PATH seems
important.

Having to patch, run and redirect the output of a shell command during
the build to make it compile was a fun excercise, as was adding the
PATH wrapper. :-)

Descriptions are my own, inspired by the originals on the homepages,
as the originals are maybe a bit too self-promoting. :-)