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.

Handle null encoding in mb_http_input()

Fix test file encoding

The mb_http_input_pass.phpt was intended to use the same encoding
as mb_http_input.phpt, not UTF-8.

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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