Fix various common mispellings.

Mostly these are just comments but there are a few in documentation
and a handful in code and tests. Hopefully this doesn't cause too much
unnecessary pain for backpatching. I relented from some of the most
common like "thru" for that reason. The rest don't seem numerous
enough to cause problems.

Thanks to Kevin Lyda's tool https://pypi.python.org/pypi/misspellings

src/test/modules/test_ddl_deparse/README

@@ -4,5 +4,5 @@ demonstration of how to use the datatype, and to provide some unit tests for
 it.
 
 The functions in this extension are intended to be able to process some
-part of the struct and produce some readable output, preferrably handling
+part of the struct and produce some readable output, preferably handling
 all possible cases so that SQL test code can be written.

src/test/regress/expected/event_trigger.out

@@ -30,14 +30,14 @@ create event trigger regress_event_trigger_end on ddl_command_end
    execute procedure test_event_trigger();
 -- should fail, food is not a valid filter variable
 create event trigger regress_event_trigger2 on ddl_command_start
-   when food in ('sandwhich')
+   when food in ('sandwich')
    execute procedure test_event_trigger();
 ERROR:  unrecognized filter variable "food"
--- should fail, sandwhich is not a valid command tag
+-- should fail, sandwich is not a valid command tag
 create event trigger regress_event_trigger2 on ddl_command_start
-   when tag in ('sandwhich')
+   when tag in ('sandwich')
    execute procedure test_event_trigger();
-ERROR:  filter value "sandwhich" not recognized for filter variable "tag"
+ERROR:  filter value "sandwich" not recognized for filter variable "tag"
 -- should fail, create skunkcabbage is not a valid command tag
 create event trigger regress_event_trigger2 on ddl_command_start
    when tag in ('create table', 'create skunkcabbage')

src/test/regress/sql/event_trigger.sql

@@ -32,12 +32,12 @@ create event trigger regress_event_trigger_end on ddl_command_end
 
 -- should fail, food is not a valid filter variable
 create event trigger regress_event_trigger2 on ddl_command_start
-   when food in ('sandwhich')
+   when food in ('sandwich')
    execute procedure test_event_trigger();
 
--- should fail, sandwhich is not a valid command tag
+-- should fail, sandwich is not a valid command tag
 create event trigger regress_event_trigger2 on ddl_command_start
-   when tag in ('sandwhich')
+   when tag in ('sandwich')
    execute procedure test_event_trigger();
 
 -- should fail, create skunkcabbage is not a valid command tag