user_thread.h: use struct keyword to be usable in C.

* Introduced new header directory headers/private/system which is supposed
to contain headers shared by kernel and userland (mainly libroot).
* Moved quite a few private kernel headers to the new location. Split
several kernel headers into a shared part and one that is still kernel
private. Adjusted all affected Jamfiles and source in the standard x86
build accordingly. The build for other architectures and for test code
may be broken.
* Quite a bit of userland code still includes private kernel headers.
Mostly those are <util/*> headers. The ones that aren't strictly
kernel-only should be moved to some other place (maybe
headers/private/shared/util).


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@25486 a95241bf-73f2-0310-859d-f6bbb57e9c96

* For each userland team the kernel creates an area in the userland
address space that is fully locked and marked B_KERNEL_AREA. It can
thus be accessed by the kernel without additional checks.
* For each userland thread we do create a user_thread structure in that
area. The structure is accessible from userland via TLS, using the
private get_user_thread() function.
* Introduced private userland functions [un]defer_signals(). They can be
used to cheaply disable/re-enable signal delivery. They use the
user_thread::defer_signals/pending_signals fields which are
checked/updated by the kernel.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@25451 a95241bf-73f2-0310-859d-f6bbb57e9c96

* Introduced new header directory headers/private/system which is supposed
to contain headers shared by kernel and userland (mainly libroot).
* Moved quite a few private kernel headers to the new location. Split
several kernel headers into a shared part and one that is still kernel
private. Adjusted all affected Jamfiles and source in the standard x86
build accordingly. The build for other architectures and for test code
may be broken.
* Quite a bit of userland code still includes private kernel headers.
Mostly those are <util/*> headers. The ones that aren't strictly
kernel-only should be moved to some other place (maybe
headers/private/shared/util).


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@25486 a95241bf-73f2-0310-859d-f6bbb57e9c96

* For each userland team the kernel creates an area in the userland
address space that is fully locked and marked B_KERNEL_AREA. It can
thus be accessed by the kernel without additional checks.
* For each userland thread we do create a user_thread structure in that
area. The structure is accessible from userland via TLS, using the
private get_user_thread() function.
* Introduced private userland functions [un]defer_signals(). They can be
used to cheaply disable/re-enable signal delivery. They use the
user_thread::defer_signals/pending_signals fields which are
checked/updated by the kernel.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@25451 a95241bf-73f2-0310-859d-f6bbb57e9c96