Added option to disable prepared statements for postgresql

[sabudaye]

#1316

[weiss]

I didn't look into the actual change, just some random comments:

• According to Travis, the build currently fails with src/ejabberd_sql.erl:526: variable 'Host' is unbound.
• The ejabberd.yml.example diff contains many lines where only whitespace is changed.
• I'd rather name the option sql_prepared_statements and document that it currently only affects PostgreSQL.

[coveralls]

Coverage increased (+0.009%) to 31.833% when pulling 026394a on sabudaye:pg_prepared_statements into 9cee376 on processone:master.

[sabudaye]

@weiss fixed. Not sure about ?MYNAME macros, but it was already used in ejabberd_auth_* modules in similar case

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[p1bot]

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