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

doc/src/sgml/config.sgml

@@ -2518,7 +2518,7 @@ include_dir 'conf.d'
         less than <varname>wal_writer_flush_after</> bytes of WAL have been
         produced since, WAL is only written to the OS, not flushed to disk.
         If <varname>wal_writer_flush_after</> is set to <literal>0</> WAL is
-        flushed everytime the WAL writer has written WAL.  The default is
+        flushed every time the WAL writer has written WAL.  The default is
         <literal>1MB</literal>. This parameter can only be set in the
         <filename>postgresql.conf</> file or on the server command line.
        </para>

doc/src/sgml/pgcrypto.sgml

@@ -866,7 +866,7 @@ Applies to: pgp_sym_encrypt
     be a value between 1024 and 65011712, inclusive.
    </para>
 <literallayout>
-Default: A random value bewteen 65536 and 253952
+Default: A random value between 65536 and 253952
 Applies to: pgp_sym_encrypt, only with s2k-mode=3
 </literallayout>
   </sect4>

doc/src/sgml/ref/psql-ref.sgml

@@ -4168,7 +4168,7 @@ testdb=&gt; <userinput>\crosstabview first second</userinput>
 </programlisting>
 
 This second example shows a multiplication table with rows sorted in reverse
-numerical order and columns with an independant, ascending numerical order.
+numerical order and columns with an independent, ascending numerical order.
 <programlisting>
 testdb=&gt; <userinput>SELECT t1.first as "A", t2.first+100 AS "B", t1.first*(t2.first+100) as "AxB",</userinput>
 testdb(&gt; <userinput>row_number() over(order by t2.first) AS ord</userinput>