rename variables, struct fields and functions to include an "x" when they can panic for handling errors

and document the convention in develop.txt.
spurred by running errcheck again (it has been a while). it still has too many
false to enable by default.

imapclient/parse.go

@@ -856,8 +856,7 @@ func (c *Conn) xliteral() []byte {
 		c.xerrorf("refusing to read more than 1MB: %d", size)
 	}
 	if sync {
-		_, err := fmt.Fprintf(c.bw, "+ ok\r\n")
-		c.xcheckf(err, "write continuation")
+		fmt.Fprintf(c.xbw, "+ ok\r\n")
 		c.xflush()
 	}
 	buf := make([]byte, int(size))