Use roken_get_*() instead of getpwuuid()

Using non-reentrant getpwuid() (or getpwnam(), or getspnam())  can be
dangerous.  We had a report of a login application / PAM that calls
those, and Heimdal, by calling them too, clobbered the cached struct
passwd used by the login app / PAM.

configure.ac

@@ -524,6 +524,7 @@ KRB_CAPABILITIES
 rk_DLADDR
 
 AC_CHECK_GETPWNAM_R_POSIX
+AC_CHECK_GETPWUID_R_POSIX
 
 dnl detect doors on solaris
 if test "$enable_pthread_support" != no; then