implement arch+= and arch-= for sources.list

Default is to acquire all architectures from APT::Architectures which
can be changed by arch=, but this isn't very flexible if you want
"mostly" the default as you have to hardcode the architectures then,
so arch-= and arch+= can be used to add/remove architectures from the
default set.

On a machine with 'amd64' and 'i386' configured the lines:
 deb [arch+=armel] http://example.org/debian wheezy rocks
 deb [arch-=amd64] http://example.org/debian jessie rocks
will result in the download of:
 wheezy Packages for 'amd64', 'i386' and 'armel'
 jessie Packages for 'i386'

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