Avoid sharing cygheaps across Cygwin versions

It frequently leads to problems when trying, say, to call from Git for
Windows' Bash into Cygwin's or MSYS2's, merely because sharing that data
is pretty finicky.

For example, using the Git for Windows that is current at time of
writing, trying to call MSYS2's Bash from Git for Windows' Bash fails
somewhere in `_cmalloc()`, without any error message, and with the
rather misleading exit code 127 (a code which is reserved to indicate
that a command was not found).

Let's just treat these as completely incompatible with one another, by
virtue of using a different `CHILD_INFO_MAGIC` constant.

One consequence is that spawned MSYS processes using a different MSYS2
runtime will not be visible as such to the parent process, i.e. they
cannot share any resources such as pseudo terminals. But that's okay,
they are simply treated as if they were regular Win32 programs.

This should also help the scenario where interactions between two
different versions of Git for Windows lead to those infamous `cygheap
base mismatch detected` problems mentioned e.g. in
git-for-windows/git#4255

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/dcrt0.cc

@@ -530,7 +530,7 @@ get_cygwin_startup_info ()
   child_info *res = (child_info *) si.lpReserved2;
 
   if (si.cbReserved2 < EXEC_MAGIC_SIZE || !res
-      || res->intro != PROC_MAGIC_GENERIC || res->magic != CHILD_INFO_MAGIC)
+      || res->intro != PROC_MAGIC_GENERIC || res->magic != (CHILD_INFO_MAGIC ^ CYGWIN_VERSION_DLL_COMBINED))
     {
       strace.activate (false);
       res = NULL;

winsup/cygwin/sigproc.cc

@@ -811,7 +811,7 @@ int child_info::retry_count = 0;
 child_info::child_info (unsigned in_cb, child_info_types chtype,
 			bool need_subproc_ready):
   msv_count (0), cb (in_cb), intro (PROC_MAGIC_GENERIC),
-  magic (CHILD_INFO_MAGIC), type (chtype), cygheap (::cygheap),
+  magic (CHILD_INFO_MAGIC ^ CYGWIN_VERSION_DLL_COMBINED), type (chtype), cygheap (::cygheap),
   cygheap_max (::cygheap_max), flag (0), retry (child_info::retry_count),
   rd_proc_pipe (NULL), wr_proc_pipe (NULL), sigmask (_my_tls.sigmask)
 {