Escape sort strings to support function sorts like geodist()

parseSortString() in Solr_Base_Query.php uses preg_match() to check against available sorts, like so (around line 540):

$fields = implode('|', array_keys($this->available_sorts));
if (preg_match('/^(?:(' . $fields . ') (asc|desc),?)+$/', $sortstring, $matches)) {

Nowhere are available_sorts escaped however, so attempting to sort on a function like geodist() doesn't work immediately like you would assume it should. For example, this doesn't work because the parens are applied to the regex without being escaped:

$query->setAvailableSort('geodist()', array('title' => 'Distance', 'default' => 'asc'));
$query->setSolrsort('geodist()', 'asc');

Whereas this does:

$query->setAvailableSort(preg_quote('geodist()'), array('title' => 'Distance', 'default' => 'asc'));
$query->setSolrsort('geodist()', 'asc');

This escaping should probably happen somewhere in Solr_Base_Query though.

Note: Since none of the core sorts have parens and geospatial querying is still a mostly custom thing, this is definitely low priority.