tsock
parasite: move code about parasite initialization in a separate function

The current scheme is used two commands to daemonize parasite.
The first command blocks signals, then a sigframe generated and the
second command daemonizes parasite. If CRIU crashes between this
commands, signals will not be unlocked.

PTRACE_GETSIGMASK allows to do the same with one command and a window,
when a process state can't be restored, is smaller.
parasite: block signals for each parasite command

Pending signals should be saved, so signals should be blocked.
Signals are blocked for EACH command, because a chance to corrupt a
process state should be so small as possible.
parasite: use only one command for executing parasite in a daemon mode

If a kernel supports PTRACE_SETSIGMASK, criu don't need to execute
PARASITE_CMD_INIT and PARASITE_CMD_DAEMONIZE, because the frist command
is used only for blocking signals. If criu crashes between these
commands, a process state will be corrupted, because all signals remain
blocked.
parasite: restore registers after each parasite command

Currenly we can restore blocking signals with help ptrace,
so if registers are restored after command, a process state
will not be corrupted if criu crashes.