signal-util: don't introduce symbols with double underscores

signal-util: don't introduce symbols with double underscores

ANSI C reserves identifiers beginning with an underscore for compiler
internal stuff. We already invade that namespace plenty and probably
should not. But even going for the doubly underscore prefixed namespace
is a bit too much. Let's just rename the offending table as
"static_signal_table[]", since it lists the static defined signals
rather than the "dynamic" RTSIGMIN/RTSIGMAX signals.