Simplify windows-nat.c::get_module_name

Simplify windows-nat.c::get_module_name

Now that get_module_name is no longer called for handling DLL events,
we can simplify it a bit, knowing that the only use is to get the
executable's filename.

While doing so, we adjusted the implementation a bit to avoid
references to DLLs, renamed it to make its more-targeted usage
more explicit, moved it right before the only function that uses it.
We also remove the use of hard-coded length for the buffers being
used.

gdb/ChangeLog:

	* windows-nat.c (get_module_name): Delete.
	(windows_get_exec_module_filename): New function, mostly
	inspired from get_module_name.
	(windows_pid_to_exec_file): Replace call to get_module_name
	by call to windows_get_exec_module_filename.