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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