Add comments to pagecache_impl_t
Make utf8 modul safe

The function decodechar_utf8 now takes the size of the buffer as
argument to check against buffer overflow.

The function skipchar_utf8 now checks the encoding and returns an
error if the skipped bytes are not encoded corrrectly.

Implement function encodechar_utf8 which does the reverse of decodechar.
Rename project testunit into unittest
Make test/static/init.sh work

Some files are adapted so that C-kern/test/static/init.sh
does not issue any errors.
Implement Thread Local Storage

The module thread_tls.c is the result of the refactoring of
stackframe implementation from module thread.

It allocates memory for all thread local variables but does not
use it. The replacement of all variables declared with __thread
as part of the thread local storage will be done in a future step.
Rename KONFIG values into KONIFG_lowercase

Configuration variables are named KONFIG_UPPERCASE
and their assigned values are renamed from de,en,THREAD,SYSUSER,...
into KONFIG_de,KONFIG_lowercase,...