Implement extended iscsi: rfc4173 syntax which allows specifying credentials

Implement extended iscsi: rfc4173 syntax which allows specifying credentials

As discussed before, it would be nice to be able to specify
the iscsi chap credentials inside the netroot=iscsi:.....
syntax, this patch implements this in a backwards compatible way, like
this:
iscsi:username:pass@127.0.0.1::3260::iqn.2009-01.com.example:testdisk
iscsi:username:pass:reverse:pass@127.0.0.1::3260::iqn.2009-01.com.example:test

The only downside is that the backwards compatibility is broken when there
is an @ in the iscsi target name (very unlikely), that can still be used,
but only like this:
iscsi:@192.168.1.100::3260::iqn.2009-01.com.example:testdi@sk