返回当前或指定驱动器的当前目录.
(dos_pwdir [drive])
drive
盘符 (for example, C:, D:).
如果成功, 返回指定驱动器的当前目录.
nil 失败时.
Command: (dos_pwdir)
"c:\\temp\\"
Command: (dos_pwdir "D:")
"d:\\projects\\drawings\\"