Read-only attribute versus read permissions

[HertzDevil]

On Windows, a file has read permissions if it doesn't have the read-only file attribute set:

crystal/src/crystal/system/win32/file_info.cr

Lines 34 to 39 in 62f27b2

	def system_permissions : ::File::Permissions
	if @file_attributes.dwFileAttributes.bits_set? LibC::FILE_ATTRIBUTE_READONLY
	permissions = ::File::Permissions.new(0o444)
	else
	permissions = ::File::Permissions.new(0o666)
	end

In fact, this attribute completely determines whether a File::Permissions has the read bits, because #13233 isn't implemented yet.

BSD-like systems, including Darwin, also have several file attributes in the LibC::Stat#st_flags that may make a file read-only. All have UF_IMMUTABLE = 0x00000002 and SF_IMMUTABLE = 0x00020000. FreeBSD additionally has UF_READONLY = 0x00001000, that maps to the Windows attribute above, but that attribute seems to have no real effect. Should those flags be reflected in File::Info#permissions? Should Windows do it in the first place?