ppp: Don't fetch a non-existent variadic argument

[jhb]

Summary

Only fetch the optional mode argument to ID0open to pass to open(2) if
O_CREAT is present in the flags argument. It is UB to fetch an
argument that doesn't exist. On CHERI this UB results in a fault.

Obtained from: CheriBSD
Sponsored by: AFRL, DARPA