Add type for $_tlb->process_environment_block->process_parameters

The type then looks like this:

(gdb) pt $_tlb->process_environment_block->process_parameters
type = struct rtl_user_process_parameters {
    DWORD32 maximum_length;
    DWORD32 length;
    DWORD32 flags;
    DWORD32 debug_flags;
    void *console_handle;
    DWORD32 console_flags;
    void *standard_input;
    void *standard_output;
    void *standard_error;
    unicode_string current_directory;
    void *current_directory_handle;
    unicode_string dll_path;
    unicode_string image_path_name;
    unicode_string command_line;
    void *environment;
    DWORD32 starting_x;
    DWORD32 starting_y;
    DWORD32 count_x;
    DWORD32 count_y;
    DWORD32 count_chars_x;
    DWORD32 count_chars_y;
    DWORD32 fill_attribute;
    DWORD32 window_flags;
    DWORD32 show_window_flags;
    unicode_string window_title;
    unicode_string desktop_info;
    unicode_string shell_info;
    unicode_string runtime_data;
} *

It's mainly useful to get the current directory, or the full command line:

(gdb) p $_tlb->process_environment_block->process_parameters->current_directory
$1 = {
  length = 26,
  maximum_length = 520,
  buffer = 0xe36c8 L"C:\\src\\tests\\"
}
(gdb) p $_tlb->process_environment_block->process_parameters->command_line
$2 = {
  length = 94,
  maximum_length = 96,
  buffer = 0xe32aa L"\"C:\\gdb\\build64\\gdb-git\\gdb\\gdb.exe\" access.exe"
}

The type names are all lowercase because the existing types created
by windows_get_tlb_type are also lowercase.

Type unicode_string is documented at [1].
The official documentation [2] for rtl_user_process_parameters is limited,
so I've used this other page [3].

[1] https://docs.microsoft.com/en-us/windows/win32/api/ntdef/ns-ntdef-_unicode_string
[2] https://docs.microsoft.com/en-us/windows/win32/api/winternl/ns-winternl-rtl_user_process_parameters
[3] https://www.nirsoft.net/kernel_struct/vista/RTL_USER_PROCESS_PARAMETERS.html

gdb/ChangeLog:

2020-01-16  Hannes Domani  <ssbssa@yahoo.de>

	* windows-tdep.c (windows_get_tlb_type):
	Add rtl_user_process_parameters type.