build-sys: tell Makefile.am to distribute kernel-install
build-sys: fix update-man-rules for vpath builds
man: drop rhs parts in snippets in directive index

-       --exit-if-exists=file
+       --exit-if-exists=
etc.
nspawn: environment would be truncated with TERM unset
systemd-activate: add a socket-activation test tool
man: document systemd-nspawn behaviour with -b

Cf. cb96a2c69 and 1ddf879a.