Rename "target child" to "target native".

Rename "target child" to "target native".

I had been pondering renaming "target child" to something else.
"child" is a little lie in case of "attach", and not exactly very
clear to users, IMO.  By best suggestion is "target native".  If I
were to explain what "target child" is, I'd just start out with "it's
the native target" anyway.  I was worrying a little that "native"
might be a lie too if some port comes up with a default target that
can run but is not really native, but I think that's a very minor
issue - we can consider that "native" really means the default built
in target that GDB supports, instead of saying that's the target that
debugs host native processes, if it turns out necessary.

This change doesn't affect users much, because "target child" results
in error today:

 (gdb) target child
 Use the "run" command to start a child process.

Other places "child" is visible:

 (gdb) help target
 ...
 List of target subcommands:

 target child -- Child process (started by the "run" command)
 target core -- Use a core file as a target
 target exec -- Use an executable file as a target
 ...

 (gdb) info target
 Symbols from "/home/pedro/gdb/mygit/build/gdb/gdb".
 Child process:
	 Using the running image of child Thread 0x7ffff7fc9740 (LWP 4818).
	 While running this, GDB does not access memory from...
 ...

These places will say "native" instead.  I think that's a good thing.

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

	* inf-child.c (inf_child_open): Remove mention of "child".
	(inf_child_target): Rename target to "native" instead of "child".

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

	* gdb.base/default.exp: Test "target native" instead of "target
	child".