Use fully qualified ::sta::float_inf - Tcl9 lookup changed.

[hzeller]

To get the value of a global variable, it seems in Tcl9, this requires to fully qualify it with leading ::.

While at it, make float_inf readonly, as it certainly is not meant to be modified.

Thanks to @mikesinouye for spotting the problem.

Fixes: #422

[hzeller]

With that, the example from #422 works with both, tcl-8 and tcl-9

$ cat ~/foo.tcl | build-8/sta -no_splash
Tcl patchlevel = 8.5.19
sta::float_inf = 1.0000000150474662e+30
Error: 1571 No network has been linked.

$ cat ~/foo.tcl | build-9/sta -no_splash
Tcl patchlevel = 9.0.3
sta::float_inf = 1.0000000150474662e+30
Error: 1571 No network has been linked.

[jjcherry56]

I added variable commands to import the variables instead