pdopenpid(2): in cap mode, translate all errors from pdopenpid1() to ECAPMODE

to not leak information about unused pids or system processes' pids.

Reviewed by:	markj
Fixes:	73c92a978c ("pdopenpid(2): allow in capability mode with restrictions")
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59252

sys/kern/sys_procdesc.c

@@ -728,7 +728,10 @@ kern_pdopenpid(struct thread *td, pid_t pid, int flags)
 			fp->f_pdflags |= F_PD_NOKILL | F_PD_NOFINSTALL;
 			filecaps_free(&fcaps);
 		}
+	} else if (IN_CAPABILITY_MODE(td)) {
+		error = ECAPMODE;
 	}
+
 	fdrop(fp, td);
 
 	if (pdf != NULL) {