Add test case for multithreaded simultaneous auth
Make libradauth threadsafe
Work around nasty select() "undefined behaviour"

selet() only allows fd's *smaller* than 1024 in any fd_set. Doing an
FD_SET(2048, &set) will simply write to some memory area outside the
set.

This actually occured because the sockfd was not properly closed on
exit; thus the fd count increased, eventually leading to an
(uncontrollable) stack overflow, causing random segfaults.

This should be fixed now. The question remains: What if we get back an
fd >= 1024? Right now, the program will simply assert(sockfd < 1024).
Properly document re-entrant usage and re-work example
By default, link test programs against *local* library
Merge branch 'reentrancy'

* reentrancy:
  By default, link test programs against *local* library
  Properly document re-entrant usage and re-work example
  Work around nasty select() "undefined behaviour"
  Make libradauth threadsafe
  Add test case for multithreaded simultaneous auth
Initialize errmsg before (eventually) writing to it
Correct debugging statement
Instead of assert(sockfd < FD_SETSIZE), issue a fatal warning

Really, this is a minor condition and a simple library should not crash
a server because there are ressource shortages.