Add a portable way to get/set charset of a connection/query/result ([http://pear.php.net/bugs/bug.php?id=4666 Request #4666 Different database charsets])

There are four (or more) possible settings related to character set/encoding.
# client: charset of a query, as it leaves from the client
# connection (server): charset of a query, as the server receives
# database/table/column: charset/collation of a database/table/field, as it's kept in the server
# results (message): charset of a result, as it leaves from the server

**It seems for most RDBMS there is only 1 setting, which sets the client charset. The database charset is determined when the database/table is setup. Its the job of the client-server combination to convert the charsets at some point. This can be set inside an array dsn using the "charset" key.**

**What is mainly missing at this point is support for setting database, table, column and result level charsets and collations.**

Collation (sorting) is also another parameter to be considered.

+++ Where to get/set them ?
Possible places:
* (set) a parameter in DSN ?
* (set) an option in MDB2::connect ?
 * a natural place for connection charset setting ?
* (get/set) a new method in MDB2 ?
* (get/set) a new method in MDB2_Result ?
* (get/set) a new method in MDB2_Datatype ?
* (schema) new <charset> tag in the xml schema (database, table, field) ?

+++ Survey of DBMS-specific commands

* MySQL: [http://dev.mysql.com/doc/refman/5.0/en/charset-connection.html]
 * client: **SET character_set_client = charset;**
 * connection: **SET character_set_connection = charset;**
 * database: **CREATE DATABASE database DEFAULT CHARACTER SET charset COLLATE collation;**
 * table: **CREATE TABLE table1 (...) CHARACTER SET charset COLLATE collation;**
 * field: **CREATE TABLE table1 (column1 VARCHAR(5) CHARACTER SET charset COLLATE collation);**
 * results: **SET character_set_results = charset;**
  * if no conversion needed, set it to NULL (e.g. SET character_set_results = NULL;)
 * Shortcuts:
  * **SET NAMES 'charset'**
   * equivalent to these three commands:
    * SET character_set_client = charset;
    * SET character_set_results = charset;
    * SET character_set_connection = charset;
   * setting character_set_connection to x also sets collation_connection to the default collation for x.
  * **SET CHARACTER SET charset**
   * equivalent to these three commands:
    * SET character_set_client = charset;
    * SET character_set_results = charset;
    * SET **collation_connection** = **@@collation_database**;
   * similar to SET NAMES, but sets the connection character set and collation to be those of the **default** database.
* SQLite: [http://www.sqlite.org/pragma.html]
 * client:
 * connection:
 * database:
 * table: **PRAGMA encoding="charset";**
  * only effective for newly created table, cannot be changed later
 * results:
* PostgreSQL: [http://www.postgresql.org/docs/8.1/interactive/multibyte.html]
 * client: **SET CLIENT_ENCODING TO 'value';**
 * connection: -
 * database: **CREATE DATABASE korean WITH ENCODING 'EUC_KR';**
 * table: 
 * field: 
 * results: **SET CLIENT_ENCODING TO 'value';**
  * if no conversion needed **RESET client_encoding;**
 * Shortcuts:
  * **SET NAMES 'charset'** (standard sql)
   * equivalent to :
    * **SET CLIENT_ENCODING TO 'value';**
* Oracle: [http://www.oracle.com/technology/tech/globalization/htdocs/nls_lang%20faq.htm]
 * client: oci_connect() or NLS_LANG environmental variable
 * connection: -
 * database:
 * table:
 * results:
* Interbase/Firebird: [http://www.destructor.de/firebird/charsets.htm]
 * client: ibase_connect() or ibase.default_charset ini setting
 * connection: N/A
 * database: **CREATE DATABASE korean DEFAULT CHARACTER SET 'EUC_KR';**
 * table: N/A
 * field: **CREATE TABLE Table1 (column1 VARCHAR(5) CHARACTER SET charset COLLATE collation);** (NB: only for CHAR and VARCHAR fields, not blobs)
 * results:
 * Shortcuts:
  * **SET NAMES 'charset'** (standard sql)