Add PHP_IPC_PRIVATE constant

[cmb69]

This constant makes IPC_PRIVATE available to userland, which is
useful for shmop_open() and sem_get(), for instance.

[nikic]

I don't think we want to expose this constant directly from core. It should be part of the shm exts (possibly defined by both, if not yet defined).

[nikic]

Or maybe defined by both under different prefixes, dunno.

[cmb69]

Well, it's actually about (at least) three extensions. Not sure, if I like how I've done it now (might be confusing, especially for the docs). I think I wouldn't like three differently named constants either.

[nikic]

I don't really have a good suggestion of how to handle this :(

[cmb69]

I've mailed internals.

[bishopb]

The conditional definition from commit a33d2f3 seems reasonable to me.

Perhaps the constant should be named PHP_IPC_NEW, per guidance from the man page:

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function

[cmb69]

Thinking about this again, I do no longer think it is a good idea to use the same name. This can bite us, if, for instance, another extension defines the constant with another value. The proper way forward would be to define different constants for the different extensions, but apparently there is not much interested in having these, so I'm closing this PR.