Win32: removed attempt to use a drive letter in ngx_fs_bsize().

Win32: removed attempt to use a drive letter in ngx_fs_bsize().

Just a drive letter might not correctly represent file system being used,
notably when using symlinks (as created by "mklink /d").  As such, instead
of trying to call GetDiskFreeSpace() with just a drive letter, we now always
use GetDiskFreeSpace() with full path.

Further, it looks like the code to use just a drive letter never worked,
since it tried to test name[2] instead of name[1] to be ':'.