Convert variable 'theme_link' to config system

Here we go.

l() can either render links inline, or pass through theme_link() Rendering a link inline in l() is faster than using theme_link(). Because l() is so heavily used, using theme_link() can add a lot of overhead to page views, so by setting theme_link to FALSE you can disable theme_link entirely.

Here's the inline comment:

// Determine if rendering of the link is to be done with a theme function
// or the inline default. Inline is faster, but if the theme system has been
// loaded and a module or theme implements a preprocess or process function
// or overrides the theme_link() function, then invoke theme(). Preliminary
// benchmarks indicate that invoking theme() can slow down the l() function
// by 20% or more, and that some of the link-heavy Drupal pages spend more
// than 10% of the total page request time in the l() function.
if (!isset($use_theme) && function_exists('theme')) {
    // Allow edge cases to prevent theme initialization and force inline link
    // rendering.
    if (variable_get('theme_link', TRUE)) {

You can see this comment for effulgentsia's numbers on this.

Moved into system.performance.

Um, don't know what is up with that interdiff :/

OK, will reroll.

Rerolled against current head, bumped update number even higher.

The same test is failing here, but I have rerolled with a new system update in the meantime. Investigating.

Thanks to timplunkett for the help!

Here's an interdiff of the changes to the test that was failing before.