In many cases, binaries can no longer dump core after calling setuid().

Where the PR_SET_DUMPABLE macro is set, use the prctl() system call 
to tell the kernel that it's allowed to dump the core of the server.


configure.in:
  Test system for "sys/prctl.h", to get access to prctl().
sql/mysqld.cc:
  Add a process-control operation that tells the Linux kernel that it 
  is allowed to dump core after setuid().