skalibs: new package

Skalibs is a set of general-purpose C programming libraries used for
building all software at skarnet.org.
execline: new package

Execline is a small scripting language, to be used in place of a shell
in non-interactive scripts.
s6: new package

s6 is skarnet.org's small & secure supervision software suite.

The s6-log program is modified to call execlineb directly rather than
searching PATH.
Merge pull request #3725 from pmahoney/s6

S6