Log and report rtld error messages via utrace(2)/ktrace(1).

[peterj]

Summary

Extend LD_UTRACE by also generating utrace(2) log events for runtime linker
errors.

MFC after: 1 week

Test Plan

Compile the following code and execute with

LD_UTRACE=yes ktrace ./a.out ; kdump -t u

Note the inclusion of error messages.

---

#include <dlfcn.h>
#include <err.h>
#include <stdio.h>

int main(int argc, char **argv)
{

if (dlsym(NULL, "symbol_missing") != NULL)
   errx(1, "Found unexpected symbol");
return (0);

}