libpq: Add target_session_attrs parameter.

Commit 274bb2b385 made it possible to
specify multiple IPs in a connection string, but that's not good
enough for the case where you have a read-write master and a bunch of
read-only standbys and want to connect to whichever server is the
master at the current time.  This commit allows that, by making it
possible to specify target_session_attrs=read-write as a connection
parameter.

There was extensive discussion of the best name for the connection
parameter and its values as well as the best way to distinguish master
and standbys.  For now, adopt the same solution as JDBC: if the user
wants a read-write connection, issue 'show transaction_read_only' and
rejection the connection if the result is 'on'.  In the future, we
could add additional values of this new target_session_attrs parameter
that issue different queries; or we might have some way of
distinguishing the server type without resorting to an SQL query; but
right now, we have this, and that's (hopefully) a good start.

Victor Wagner and Mithun Cy.  Design review by Álvaro Herrera, Catalin
Iacob, Takayuki Tsunakawa, and Craig Ringer; code review by me.  I
changed Mithun's patch to skip all remaining IPs for a host if we
reject a connection based on this new parameter, rewrote the
documentation, and did some other cosmetic cleanup.

Discussion: http://postgr.es/m/CAD__OuhqPRGpcsfwPHz_PDqAGkoqS1UvnUnOnAB-LBWBW=wu4A@mail.gmail.com

src/interfaces/libpq/libpq-fe.h

@@ -62,7 +62,9 @@ typedef enum
 								 * backend startup. */
 	CONNECTION_SETENV,			/* Negotiating environment. */
 	CONNECTION_SSL_STARTUP,		/* Negotiating SSL. */
-	CONNECTION_NEEDED			/* Internal state: connect() needed */
+	CONNECTION_NEEDED,			/* Internal state: connect() needed */
+	CONNECTION_CHECK_WRITABLE	/* Check if we could make a writable
+								 * connection. */
 } ConnStatusType;
 
 typedef enum