* Makefile.in: Autogenerate some header files which provide magic numbers.

* Makefile.in: Autogenerate some header files which provide magic numbers.
Force dependencies for files which depend on autogenerated headers to ensure
that they are always built.
* child_info.h (child_info): Add new fields to accommodate new magic number
header stuff.
* dcrt0.cc: Rely on "child_info_magic.h" to ensure that correct child_info
magic numbers are used.
(dll_crt0_1): Temporarily remove _cygwin_testing_magic test.
(_dll_crt0): Do more testing on magic numbers from fork_info structure.  Call
"multiple_cygwin_problem" where appropriate.
(multiple_cygwin_problem): Rename from multiple_cygwin_die.  Issue a warning or
die, as appropriate based on cygwin version/magic number mismatch.
* pinfo.cc (pinfo::exit): Don't attempt to dereference `this' if it doesn't
exist.  This can happen when a fatal error occurs early in process
initialization.
* shared.cc: Rely on "shared_info_magic.h" to accommodate that new magic number
header stuff.
(shared_info::initialize): Use new magic number stuff, for shared region.
(memory_init): Ditto, for mount table.
* shared_info.h: Accomodate new magic number stuff for shared region and mount
table.
* sigproc.cc: Rely on "child_info_magic.h" to accommodate new magic number
header stuff.
(init_child_info): Initialize new fields in child_info) to accomodate magic
numbers.
* winsup.h: Rename multiple_cygwin_die to multiple_cygwin_problem.
* include/cygwin/version.h: Define macros for manipulating version magic.
* cygmagic: New shell script for generating magic numbers.