Fix some errors in mysqlnd charset list

I fixed the stuff that seemed obviously wrong, but there are some more
differences with the SHOW COLLATIONS output:

 * The whole range of "utf16" collations missing
 * "filename" missing
 * "ucs2_general_mysql500_ci" and "utf8_general_mysql500_ci" missing

I wasn't sure whether those omissions are intentional, so I didn't add
them.