Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Convert shmop resources to opaque objects

We make `shmop_close()` a NOP, and deprecate the function right away;
detaching from SHM now happens when the wrapper object is freed.

Fix #79566: Private SHM is not private on Windows

We map the POSIX semantics of `IPC_PRIVATE` by creating unnamed file
mapping objects on Windows. While that is not particularly useful

Fix #79566: Private SHM is not private on Windows

We map the POSIX semantics of `IPC_PRIVATE` by creating unnamed file
mapping objects on Windows. While that is not particularly useful for
ext/shmop, which is the only bundled extension which uses `shmget()`,
it may be useful for external extensions.

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