MFC r365284:

Always report ENOSYS in init

While rare, encountering an unimplemented system call early in init is
catastrophic and difficult to debug. Even after a SIGSYS handler is
registered, such configurations are problematic. As such, always report
such events for pid 1 (following kern.lognosys if non-zero).

Reviewed by: kevans, imp
Obtained from: CheriBSD (plus suggestions from kevans)
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26288