(where n is a decimal integer in the range [0 .. SIGRTMAX-SIGRTMIN].
As usual a leading "sig" is ignored and the strings are case independent.
Some implementations do not name the real time signals, and using
labels like RTMIN+3 can be the only way they can be manipulated,
so allow that technique (we still return the RTnn names on the inverse
translation though).
Because this is used by both kill(1) and sh(1) the kill and trap
commands both gain access to the new notation (when !SMALL).
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