Made upspeedgraph and downspeed graph scale on a common axis

[donutAnees]

Checklist

• I have described the changes
• All new code is licensed under GPLv3

Description

Before

In the current latest code of conky, downspeedgraph and upspeedgraph look the same because they're both individually scaled with respect to max value.

After

This has now been fixed and both of them are scaled on a common axis. I have tested the code, and it does not influence any other graphs.

The changes made include

1. Adding char speedgraph flag to the struct graph and struct special_node.
2. Declaring a global double maxval to keep track of the current max value across the two graphs.
3. Adding parameter char speedgraph to scan_graph method.
4. And finally checking if the current value getting added is bigger than maxval, if so we make our scale = maxval, in the graph_append method.

Fixes #1820

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[brndnmtthws]

This is great, thanks!

A bit weird that we mix bool, char, true/false, TRUE/FALSE but that's something for another PR.