Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Only allow "pass" as input/output encoding

"pass" is not a real encoding, it just means "don't perform any
conversion". Using it as an internal encoding or passing it to
any of the m

Only allow "pass" as input/output encoding

"pass" is not a real encoding, it just means "don't perform any
conversion". Using it as an internal encoding or passing it to
any of the mbstring() function will not work (and on master commonly
assert).

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Promote unsupported encoding warnings to ValueError

Use php_mb_get_encoding instead of mbfl_name2encoding to get encoding

This reduces the number of places where the error message template is used.
Also promote the mb_check_encoding() war

Use php_mb_get_encoding instead of mbfl_name2encoding to get encoding

This reduces the number of places where the error message template is used.
Also promote the mb_check_encoding() warning to ValueError and add a test to cover the behaviour.

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