psql: Fix some strange code in SQL help creation

Struct QL_HELP used to be defined as static in the sql_help.h header
file, which is included in sql_help.c and help.c, thus creating two
separate instances of the struct.  This causes a warning from GCC 6,
because the struct is not used in sql_help.c.

Instead, declare the struct as extern in the header file and define it
in sql_help.c.  This also allows making a bunch of functions static
because they are no longer needed outside of sql_help.c.

Reviewed-by: Thomas Munro <thomas.munro@enterprisedb.com>