Improve SystemdLabel highlighting pattern

The pattern for matching labels in systemd units shouldn't allow '=' to
be highlighted within these variables, this leads to more of the line
being highlighted then we actually want, for example, let's say we have
a line like this in a unit file:

	Documentation=https://bbs.archlinux.org/viewtopic.php?pid=1420279#p1420279

Ideally, the highlighting should be limited until the first '=' to look
like this (where everything between the pair of '|'s is highlighted):

	|Documentation|=https://bbs.archlinux.org/viewtopic.php?pid=1420279#p1420279

Instead however, because the old pattern allowed anything up until a '='
to match, this is what would be highlighted:

	|Documentation=https://bbs.archlinux.org/viewtopic.php?pid=|1420279#p1420279

Using '[^=]' instead of '.' in the pattern should fix this.