Refactor field name quoting

[dregad]

This is a follow-up on the discussion in #721, to track changes targeted at 5.22.0.

• get rid of global $ADODB_QUOTE_FIELDNAMES constant
• Replace use of hardcoded strings by constants in _adodb_quote_fieldname()
• Introduce new ADOConnection method setQuoteStyle()
• Rely on ADODB_ASSOC_CASE to control casing

From #721 (comment)

I believe the following should be sufficient, along with the existing $nameQuote property and a setQuoteStyle() method accepting one of the constants.

QUOTE_NONE = 0
QUOTE_SINGLE = 1 // $nameQuote . $field . $nameQuote
QUOTE_BRACKETS = 2 // $nameQuote[0] . $field . $nameQuote[1]

From #721 (comment)

Considering $nameQuote is a public property, for BC I think we should keep supporting $nameQuote as string.

Moreover, supporting both single and bracket quote styles (for DBs allowing that) would require a separate, extra operation to set the quoting tokens (e.g. $db->nameQuote = ['[', ']']; or $db->setQuoteStyle('[', ']');) which is a low-level thing that IMO should be abstracted away.

My proposed approach simplifies that by hiding the low-level logic within the driver-specific method, e.g.

// MySQL
function setQuoteStyle(int $style) {
 $this->nameQuote = $style ? '`' : '';
}

// MSSQL
function setQuoteStyle(int $style) {
 switch ($style) {
     case QUOTE_NONE: 
       $this->nameQuote = ''; // or ['"', '"']
       break;
     case QUOTE_BRACKETS: 
       $this->nameQuote = ['[', ']']; 
       break;
     case QUOTE_SINGLE: 
     default:
       $this->nameQuote = '"';  // or  ['', '']
       break;
}