Fix 326462 Refactor vgdb to isolate invoker stuff into separate module

vgdb.c contained (conditionally compiled) "invoker" code to have ptrace syscalls
used to allow gdb/vgdb to connect to a valgrind process blocked in a syscall.
This "invoker" code is ptrace based.
Not all platforms are using ptrace.
=> refactor vgdb so as allow invoker code to be added more cleanly
for non ptrace based platforms (e.g. Darwin, Solaris).

* add file vgdb.h for:
   - definition of the vgdb-invoker interface
   - common declarations between vgdb.c and vgdb-invoker implementations
* move ptrace related code from vgdb.c to new file vgdb-invoker-ptrace.c
* new file vgdb-invoker-none.c containing an empty invoker implementation
  used on platforms that do not (yet) have a invoker implementation
  (e.g. android and darwin).
* modified Makefile.am to use one of the vgdb-invoker-*.c file depending
  on the platform.
* small changes related to changing ptraceinvoker to invoker in various files:
  gdbserver_tests/make_local_links, gdbserver_tests/nlcontrolc.vgtest,
  gdbserver_tests/mcinvokeRU.vgtest, gdbserver_tests/nlsigvgdb.vgtest  
  gdbserver_tests/mcinvokeWS.vgtest, coregrind/m_gdbserver/README_DEVELOPERS

Patch from Ivo Raisr, slightly modified



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13743 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/gdbserver_tests/make_local_links b/gdbserver_tests/make_local_links
index 73a2f00..cbdfef5 100755
--- a/gdbserver_tests/make_local_links
+++ b/gdbserver_tests/make_local_links
@@ -99,11 +99,11 @@
 # if ptrace not implemented in vgdb or OS restricts the initial attach,
 # some tests would block for a loooonnnng time.
 if gdbserver_tests/vgdb --help 2>&1 |
-    grep -e 'ptrace invoker not implemented' > /dev/null
+    grep -e 'invoker not implemented' > /dev/null
 then
-    rm -f gdbserver_tests/vgdb.ptraceinvoker
+    rm -f gdbserver_tests/vgdb.invoker
 else
-    touch gdbserver_tests/vgdb.ptraceinvoker
+    touch gdbserver_tests/vgdb.invoker
 fi
 
 # cleanup the possibly big garbage previously collected output