MFC r314924:

sbin/devfs: clarify usage

- Note existence of -m option.
- Note that -s applies to rule keyword, only, by adding usage text
specifically for the `rule` and `ruleset` keywords.

Don't go into any further detail in usage(..) -- it's best that one
reads the manpage to get a better idea of how things work as there are
a number of different option-specific keywords and arguments, as well
as some rule grammar.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

o Revert a previous delta as strlcpy(3) operates with NUL-terminated
strings and cp is not. Fix logic in the original code and eliminate
core dumps on lines without '\n'.

o Replace (an incorrect) string copy gymnastics with strlcpy(3).

PR: bin/98905
Submitted by: Fabian Keil
MFC after: 1 week

Userland signed char fixes for PPC build. Problems were using a char
return for getopt() and comparing to -1, ditto with fgetc() and EOF,
and using the kg_nice value from <sys/user.h>

Submitted by: Stefan Farfeleder <stefan@fafoe.narf.at>
Reviewed by: obrien, bde (a while back)
Tested lightly on: ppc, i386, make universe

Don't depend on pollution in <machine/limits.h> (by way of
<sys/param.h>) for definition of <stdint.h> macros.

Nuke whitespace at EOL.

Implement this (quoted from the updated man page): If the first token
of a rule specification is a single dash (``-''), rules are read from
the standard input and the rest of the specification is ignored.

Introduce the DEVFS "rule" subsystem. DEVFS rules permit the
administrator to define certain properties of new devfs nodes before
they become visible to the userland. Both static (e.g., /dev/speaker)
and dynamic (e.g., /dev/bpf*, some removable devices) nodes are
supported. Each DEVFS mount may have a different ruleset assigned to
it, permitting different policies to be implemented for things like
jails.

Approved by: phk