Set commentstring and comments

Some plugins (The-NERD-Commenter is a good example of this) use
commentstring to know how to comment and uncomment lines, vim also uses
the setting to allow for folding of comments (see :help
'commentstring').

'comments' is also occasionally used, so we might as well set that too.
Improve Systemd label 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.
Add highlighting for quotes in units