Improve testlibpq3.c's example of PQexecParams() usage to include sending

Improve testlibpq3.c's example of PQexecParams() usage to include sending
a parameter in binary format.  Also, add a TIP explaining how to use casts
in the query text to avoid needing to specify parameter types by OID.
Also fix bogus spacing --- apparently somebody expanded the tabs in the
example programs to 8 spaces instead of 4 when transposing them into SGML.