[krbdev.mit.edu #2982] pre-POSIX getpw{nam,uid}_r support

[Ken Raeburn via RT]

Some pre-POSIX systems like HP-UX 10.20 have getpwnam_r and getpwuid_r
functions with four arguments, returning int.  That's a case we don't
handle at the moment.  Also, relating to ticket 2929, I think the test
for the number of arguments is too weak -- if one of the compilations
succeeds, it thinks it knows how many arguments are needed.  It should
do two compilations, and require that exactly one succeed.