Now threads restortion (and TLS as well) works.
Threads test reports the following

    2775 (main): Counter value:    3 tls_data =    1
    2775 (main): ( 0) fsgs_base 7f9597aa46f0
    2775 (main): ( 0) fsgs_base        0
    2775 (thr3): Counter value:    4 tls_data =    4
    2775 (thr3): ( 0) fsgs_base 42c57940
    2775 (thr3): ( 0) fsgs_base        0
    2775 (thr2): Counter value:    3 tls_data =    2
    2775 (thr2): ( 0) fsgs_base 42456940
    2775 (thr2): ( 0) fsgs_base        0
    2775 (thr1): Counter value:    4 tls_data =    3
    2775 (thr1): ( 0) fsgs_base 40c62940
    2775 (thr1): ( 0) fsgs_base        0
    2775 (main): Counter value:    4 tls_data =    1
    2775 (main): ( 0) fsgs_base 7f9597aa46f0
    2775 (main): ( 0) fsgs_base        0
    2775 (thr1): Counter value:    5 tls_data =    3
    2775 (thr1): ( 0) fsgs_base 40c62940
    2775 (thr1): ( 0) fsgs_base        0

as expected.

This commits merges all preliminary commits into
the final one (sigreturn branch was always experimental
and forced update).

Still some problems remain:

1) While creating threads with clone() the
   flags are to be revisited. We use some predefined
   set here but it's not really correct.

2) No setup of pids in PCB thread zone.

3) No restore of FPU.

But at least on some basic tasks restore works well.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>