MFC: r317269
Set default uid/gid to nobody/nogroup for NFSv4 mapping.

The default uid/gid for NFSv4 are set by the nfsuserd(8) daemon.
However, they were 0 until the nfsuserd(8) was run. Since it is
possible to use NFSv4 without running the nfsuserd(8) daemon, set them
to nobody/nogroup initially.
Without this patch, the values would be set by the nfsuserd(8) daemon
and left changed even if the nfsuserd(8) daemon was killed. The default
values of 0 meant that setting a group to "wheel" would fail even when
done by root.
It also adds a definition of GID_NOGROUP to sys/conf.h.

MFC r310925:
Remove unused declaration.

MFC r298890:
Make it explicit that D_MEM cdevsw d_flag is to signify that the
driver is (or behaves identically to) /dev/mem. Remove the D_MEM flag
from random drivers.

MFC r293346:
Provide yet another KPI for cdev creation, make_dev_s(9).

MFC 286974,291653:

286974:
Remove reference to non-existent kern_openat(9).

291653:
The cdevpriv_dtr_t typedef was not able to be used in a function prototype
like the various d_*_t typedefs since it declared a function pointer rather
than a function. Add a new d_priv_dtor_t typedef that declares the function
and can be used as a function prototype. The previous typedef wasn't
useful outside of the cdevpriv implementation, so retire it.

The name d_priv_dtor_t was chosen to be more consistent with cdev methods
since it is commonly used in place of d_close_t even though it is not a
direct pointer in struct cdevsw.

MFC r274366:

Add missing privilege check when setting the dump device.

Approved by: pjd, secteam (both no objections)
Sponsored by: Multiplay

MFC r277179, r277199 and r277391:
Add a kernel function to delist our kernel character devices, so that
the device name can be re-used right away in case we are destroying
the character devices in the background.

MFC r274251:
Add DEV_MODULE_ORDERED().

MFC r277179, r277199 and r277391:
Add a kernel function to delist our kernel character devices, so that
the device name can be re-used right away in case we are destroying
the character devices in the background.

MFC r274251:
Add DEV_MODULE_ORDERED().