sysctl-s in a module should be accessible only when the module is initialized

A sysctl can have a custom handler that may access data that is initialized
via SYSINIT(9) or via a module event handler (also invoked via SYSINIT).
Thus, it is not safe to allow access to the module's sysctl-s until
the initialization is performed. Likewise, we should not allow access
to teh sysctl-s after the module is uninitialized.
The latter is easy to achieve by properly ordering linker_file_unregister_sysctls
and linker_file_sysuninit.
The former is not as easy for two reasons:

• the initialization may depend on tunables which get set when sysctl-s are registered, so we need to set the tunables before running sysinit-s
• the initialization may try to dynamically add more sysctl-s under statically defined sysctl nodes

So, this change splits the sysctl setup into two phases. In the first phase
the sysctl-s are registered as before but they are disabled and hidden from
consumers. In the second phase, done after sysinit-s, normal access to the
sysctl-s is enabled.

The change should affect only dynamic module loading and unloading after
the system boot-up. Nothing changes for sysctl-s compiled into the kernel
and sysctl-s in preloaded modules.

Discussed with: hselasky, ian, jhb
Reviewed by: julian, kib
MFC after: 2 weeks
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D12545