Detecting errors of reentrant APIs: getgrgid_r and the like

Detecting errors of reentrant APIs: getgrgid_r and the like
when returning an int can either return the error in the return
value or in the errno.  We need a new per-interp variable to store
the return value instead of clobbering the errno with it.  The new
variable, PL_reentrant_retint, is only used within the reentrancy
framework, and immediately after it's set, so there should not be
similar visibility issues as for the errno.  Spotted by Edward Moy.

p4raw-id: //depot/perl@19357