sane-backends: clean up variant names

[ryandesign (Ryan Carsten Schmidt)]

The sane-backends variants use prefixes, which is not how we usually name variants in MacPorts:

$ port variants sane-backends
sane-backends has the variants:
   disable_localbackends: turn off compilation of all backends but net
     * conflicts with enable_pnmbackend with_gphoto2
   enable_pnmbackend: enable the pnm backend for testing frontends (possible security risk, see PROBLEMS file)
     * conflicts with disable_localbackends
   universal: Build for multiple architectures
   with_gphoto2: include the gphoto2 backend
     * conflicts with disable_localbackends

• "disable_localbackends" should invert its functionality and become "localbackends", default to on
• "enable_pnmbackend" should become "pnmbackend" (or even just "pnm")
• "with_gphoto2" should become "gphoto2"

When the variants are renamed, stub variants with the old variant names should stay around for "awhile" (until "all" users have upgraded) to provide a migration path. For example:

variant disable_localbackends conflicts localbackends description {Legacy compatibility variant} {}
if {[variant_isset disable_localbackends]} {
    default_variants -localbackends
} else {
    default_variants +localbackends
}
variant enable_pnmbackend requires pnmbackend description {Legacy compatibility variant} {}
variant with_gphoto2 requires gphoto2 description {Legacy compatibility variant} {}

And the revision should be increased when the variants are renamed.

[aguynamedryan+macports@…]

attempt to rearrange variants

[aguynamedryan+macports@…]

I've taken a stab at this, but I'm a little shaky on variants.