fix 319235 --db-attach=yes is broken with Yama ptrace scoping enabled
On Ubuntu systems, ptrace_scoping could forbid a process to ptrace another.
This ptrace scoping was already handled for vgdb by using SET_PTRACER
(the valgrind process must be ptraced by vgdb when it is blocked
in a syscall).
set_ptracer is however also needed when the old mechanism --db-attach=yes
is used.
The following changes are done:
* make the set_ptracer logic callable outside gdbserver
* make set_ptracer less restrictive (i.e. allow all
  processes of the user to ptrace). This removes a limitation for vgdb.
* call the set_ptracer in the child launched for --db-attach=yes
* cleaned up the ptrace scope restriction message and doc as vgdb
  is now working properly by default, even with ptrace_scope enabled.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13384 a5019735-40e9-0310-863c-91ae7b9d1cf9