Moved TLS support from the kernel to userland.

We'll do that in userland by putting a pointer on the top
of the stack now. Since the stack has a power-of-2 maximum size,
we know the stack top and can thus use that to access TLS
very fast.
The disadvantage is that we can't use the compiler support for
TLS anymore (__thread). The advantages are that it removes
quite some special cases from the kernel, works for all
architectures, with and without dynamic linking and removes
some code from the critical context-switch path.
Additionally, I've put errno on the top of the stack. Thus, it's
thread-local now, too. Note that it is separate from TLS because
we need errno to support TLS.
Provide PAGESIZE as a constant, not config-value.
Provide the standard open-flags (just renaming).
Use the standard struct stat instead of our sFileInfo.
Added a simple, but conforming stdint.h.
Provide the standard assert semantics.

Additionally, I've added sassert(), which can be used if it
should be execute with NDEBUG, too (without checking the
assertion).
Made setjmp/longjmp conform to the C standard.
Use standard signal names.
Removed unused code.
Implemented clock.
Comments for struct tm.
Better name it PAGE_SIZE in userland, too.
Moved include/sys to kernel/include.

This way, we can use include/sys to provide the files required
POSIX.
Don't use -Wshadow. Too annoying.
Moved PAGE_SIZE to arch.h.
Move closer to POSIX with dirent.h.
Don't redefine MAX_PATH_LEN.
Moved esc/mem.h to sys/mman.h.
Moved stat-related io-functions to sys/stat.h.
Removed unused and superfluous getthreadcnt() syscall.