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.

doc/src/sgml/protocol.sgml

@@ -4734,7 +4734,10 @@ StartupMessage (F)
                 set at backend start time might be listed.  Such settings
                 will be applied during backend start (after parsing the
                 command-line options if any).  The values will act as
-                session defaults.
+                session defaults.  Spaces in option values need to be escaped
+                with a backslash (<literal>\</>). A literal backslash can be
+                passed by escaping it with another backslash
+                (i.e <literal>\\</>).
 </para>
 </listitem>
 </varlistentry>