MFC r355108 and r355170:
Fix panic when loading kernel modules before root file system is mounted.
Make sure the rootvnode is always NULL checked.

Differential Revision: https://reviews.freebsd.org/D22545
PR: 241639
Sponsored by: Mellanox Technologies

MFC r325865

Properly bzero kldstat structure to prevent kernel information leak.

Security: FreeBSD-SA-17:10.kldstat
Security: CVE-2017-1088

MFC r324311: sysctl-s in a module should be accessible only when the module is initialized

Sponsored by: Panzura

MFC r298819:

sys/kern: spelling fixes in comments.

MFC r297391:

Remove some NULL checks for M_WAITOK allocations.

MFC 290728:
Export various helper variables describing the layout and size of
certain kernel structures for use by debuggers. This mostly aids
in examining cores from a kernel without debug symbols as a debugger
can infer these values if debug symbols are available.

One set of variables describes the layout of 'struct linker_file' to
walk the list of loaded kernel modules.

A second set of variables describes the layout of 'struct proc' and
'struct thread' to walk the list of processes in the kernel and the
threads in each process.

The 'pcb_size' variable is used to index into the stoppcbs[] array.

The 'vm_maxuser_address' is used to distinguish kernel virtual addresses
from user addresses. This doesn't have to be perfect, and
'vm_maxuser_address' is a cheap and simple way to differentiate kernel
pointers from simple values like TIDs and PIDs.

While here, annotate the fields in struct pcb used by kgdb on amd64
and i386 to note that their ABI should be preserved. Annotations for
other platforms will be added in the future.

MFC r264173:
Use realloc(9) instead of doing the reallocation inline.

MFC r263080:
Use correct types for sizeof() in the calculations for the malloc(9) sizes.

MFC r259587: Invoke the kld_* event handlers from linker_load_file() ...

MFC r264173:
Use realloc(9) instead of doing the reallocation inline.

MFC r263080:
Use correct types for sizeof() in the calculations for the malloc(9) sizes.

MFC r259587: Invoke the kld_* event handlers from linker_load_file() ...