man: use <constant> for various constants which look ugly with quotes
man: add CONSTANTS section to systemd.directives(7)
journalctl: highlight MESSAGE= in verbose output

When looking at verbose output, additional "work" is required to
pick out the interesting MESSAGE= lines from all the fields.

Also, show long fields in full in verbose output mode when
OUTPUT_FULL_WIDTH is specified.
killall: do not use alloca() in argument list

It is not allowed.