Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Fix #79665: ini_get() and opcache_get_configuration() inconsistency

Overriding the given INI values in modifier callbacks is not possible,
so instead of enforcing "normalized" internal v

Fix #79665: ini_get() and opcache_get_configuration() inconsistency

Overriding the given INI values in modifier callbacks is not possible,
so instead of enforcing "normalized" internal values, we just reject
the attempted changes.

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