wrapper around the getpid() and getppid() C functions

 Perl already returns the PID and PPID in variables and builtins. Linux::Pid
 forces perl to call the underlying C functions getpid() and getppid().
 .
 This is useful with multithreaded programs. Linux' C library, using the Linux
 thread model, returns different values of the PID and the PPID from different
 threads.
 .
 A known consumer of this functionality is Apache2::SizeLimit (in
 libapache2-mod-perl2).