man/logind.conf.xml: Make "man logind.conf.d" work too
man: Factor out a common snippet for .d directories and precedence

Several manpages contain duplicate text describing a standard set of .d
configuration directories, with the usual sorting, precedence,
overrides, and so on.  Factor this common text out using XInclude before
proliferating it even further.
core: Support system.conf.d and user.conf.d directories in the usual search paths
journald: Support journald.conf.d directories in the usual search paths
systemd-sleep: Support sleep.conf.d directories in the usual search paths
bootchart: Support bootchart.conf.d directories in the usual search paths
coredump: Support coredump.conf.d directories in the usual search paths
resolved: Support resolved.conf.d directories in the usual search paths
timesyncd: Support timesyncd.conf.d directories in the usual search paths
journald-remote,journal-upload: Support .d directories in the usual search paths