dump: block signals with help of ptrace

Currently crtools blocks signals from a parasite code, but for executing
the parasite code a process has to handle all pending non-blocked signals.

It is not a problem until crtools decide to support stopped tasks.

Conflicts:
	cr-dump.c
	parasite-syscall.c
	pie/parasite.c
strace
socket-ext: provide access to socket for a test user
restore registers of threads
finalization