Initial implementation of subpackages.
Based on previous mat's implementation: https://reviews.freebsd.org/D16457
To add subpackages to a port, define:
```
SUBPACKAGES= foo bar
```
and in the plist, prefix files to put in a subpackage with `@@foo@@`.
For each subpackage, some variables can be set, if needed.
* `DESCR.<subpkg>` defaults to the `pkg-descr` if `pkg-descr.<subpkg>` does not exist (with a warning)
* `COMMENT.<subpkg>` defaults to `${COMMENT} (subpkg: <subpkg>)`
* All the `PKGMESSAGE`/`PKGINSTALL`/... variable have `.<subpkg>` version, pointing to `pkg-message.<subpkg>`/`pkg-install.<subpkg>` files.
* A few options helpers `<opt>_SUBPACKAGES` and `<opt>_*_DEPENDS.<subpkg>` can be used.
* `PLIST_FILES.<subpkg>` and `PLIST_DIRS.subpkg` were added too.
Subpackages can depend on each other, a new variable has been introduced, SELF_DEPENDS, and a special subpackage name has been introduced to refer to the main package. For example:
If the foo subpackage depends on the main subpackage:
SUBPACKAGES= foo
SELF_DEPENDS.foo= main
If the main package depends on the tools subpackage:
SUBPACKAGE= tools
SELF_DEPENDS= tools