libsys: retire getnucred in favor of getepinfo

The getnucred() function was used by UDS to obtain credentials of user
processes in a form used in the UDS API, namely the ucred structure.
Since the NetBSD merge, this structure has changed drastically (aside
from being renamed to "uucred"), and it is no longer in UDS's best
interest to use this structure internally.  Therefore, getnucred() is
no longer a useful API either, and instead we directly use the
previously private getepinfo() function to obtain credentials.

Change-Id: I80bc809de716ec0a9b7497cb109d2f2708a629d5

sys/sys/ucred.h

@@ -53,9 +53,4 @@ struct uucred {
 	gid_t		cr_groups[NGROUPS_MAX];	/* groups */
 };
 
-#if defined(__minix)
-#include <minix/type.h>
-
-int getnucred(endpoint_t proc_ep, struct uucred *ucred);
-#endif /* defined(__minix) */
 #endif /* !_SYS_UCRED_H_ */