Add utmps

utmps is a library implementing utmp in a secure way. It needs 2 small daemons at runtime (running under the s6-ipcserver super-server). This set of commits does the following:

• split the s6 package, separating the necessary s6-ipcserver (and related) binaries into a small s6-ipcserver package, so that the utmps package does not depend on the whole s6 package
• fix cosmetics in the s6 package
• add a new utmps package.

The utmps daemons can be started with: rc-update add utmpd boot and rc-update add wtmpd boot (yes, they need to start in the boot runlevel; utmp functionality will not be available before they're started).

To make a package such as util-linux or busybox (implementing who) build with utmps:

• compile with: -I/usr/include/utmps (so the correct utmpx.h file can be found)
• link with: -lutmps -lskarnet (so the software is linked against libutmps.a or libutmps.so, as well as libskarnet which libutmps depends on)

.pc files for utmps and skalibs have been added so pkgconf-using software can be easily updated.