Allow escaping of option values for options passed at connection start.

Allow escaping of option values for options passed at connection start.

This is useful to allow to set GUCs to values that include spaces;
something that wasn't previously possible. The primary case motivating
this is the desire to set default_transaction_isolation to 'repeatable
read' on a per connection basis, but other usecases like seach_path do
also exist.

This introduces a slight backward incompatibility: Previously a \ in
an option value would have been passed on literally, now it'll be
taken as an escape.

The relevant mailing list discussion starts with
20140204125823.GJ12016@awork2.anarazel.de.