Native targets: Add inf-child.c:inf_child_mourn_inferior and use it.

Native targets: Add inf-child.c:inf_child_mourn_inferior and use it.

Most ports do the same thing in the tail of their mourn routine - call
generic_mourn_inferior+inf_child_maybe_unpush_target.

This factors that out to a convenience function.  More could be done,
but this converts only the really obvious ones.

Tested by building GDB on x86_64 Fedora 20, mingw32 and djgpp.  The
rest is untested, but I think a patch can't get more obvious.

gdb/
2014-05-21  Pedro Alves  <palves@redhat.com>

	* inf-child.c (inf_child_mourn_inferior): New function.
	* inf-child.h (inf_child_mourn_inferior): New declaration.
	* darwin-nat.c (darwin_mourn_inferior): Use
	inf_child_mourn_inferior.
	* gnu-nat.c (gnu_mourn_inferior): Likewise.
	* inf-ptrace.c (inf_ptrace_mourn_inferior): Likewise.
	* inf-ttrace.c (inf_ttrace_mourn_inferior): Likewise.
	* nto-procfs.c (procfs_mourn_inferior): Likewise.
	* windows-nat.c (windows_mourn_inferior): Likewise.