Arping, FormatOutput(): review/resync this copied code

Noticed in PR371.

Someone should check whether new code from ping should be ported...
For possible other code than FormatOutput() too.

And maybe better document what is copied from/to where, if that still applies.

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FormatOutput():

• r70212: copied/created this function, at least, from ping.
• PR371: va_start()/va_end() fix.

ping.c

• r72765: rewritten as PrintString(). (Superseded current PR371.)
• r73626: replaced by ConResPrintf() calls.