PC-BSD/trueos 4e6c971lib/libc/gen sysctlnametomib.c, sys/kern kern_sysctl.c

sysctl: use names instead of magic numbers.

Replace magic numbers with symbols for internal sysctl operations.
Convert in-kernel and libc consumers.

Submitted by:   Pawel Biernacki
MFC after:      1 week
Differential revision:  https://reviews.freebsd.org/D21693
DeltaFile
+33-28sys/kern/kern_sysctl.c
+12-1sys/sys/sysctl.h
+2-2lib/libc/gen/sysctlnametomib.c
+47-313 files

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