proc_set_cred(): Allow 'newcred' to have multiple references

[olce]

Summary

This is an extension needed by next commit, where some additional
reference is kept on the credentials to be set on a process in order to
keep these credentials alive even after the process lock is released (an
intervening reset of process credentials could release the reference
that the process holds).

[olce]

sys/kern/kern_prot.c

425

This slipped in by mistake, removed in my local commit.

[markj]

sys/kern/kern_prot.c

2821