Correct sysent flags for dynamically loaded syscalls.

Using the https://github.com/google/capsicum-test/ suite, the
PosixMqueue.CapModeForked test was failing due to an ECAPMODE after
calling kmq_notify(). On further inspection, the dynamically
loaded syscall entry was initialized with sy_flags zeroed out, since
SYSCALL_INIT_HELPER() left sysent.sy_flags with the default value.

Add a new helper SYSCALL{,32}_INIT_HELPER_F() which takes an
additional argument to specify the sy_flags value.

Submitted by: Siva Mahadevan <smahadevan@freebsdfoundation.org>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D11576