build/meson: fix autodetecting ifcfg_rh/ifupdown plugins

"-Difcfg_rh=false" did not work, we would always fallback to
autodetection. That is wrong, an explicit "false" should be honored.
It's also not what autotools does. Fix this.

While at it, drop "distro" variable. It's not a clear concept
that can be reused and it's unused otherwise.

Also, no longer let the autodetection be based on cross compilation.
When cross-compiling, it seems not entirely unreasonable that you cross
compile to a comparable distro, so let the autodetection be based on
what we detect on the host. In any case, a user can and is encouraged
to explicitly enable/disable the plugins via "-Difcfg_rh=" or
"-Difupdown=".

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