New port: sysutils/nsysctl: New sysctl utility with libxo and extra options

nsysctl (new sysctl) is a sysctl(8) utility clone to get or set kernel
state with libxo, sysctlmibinfo and extra options; improvements:

• sysctlmibinfo(3) provides a simple API to the sysctl MIB,
• all code about "opaque values" is in opaque.c,
• output via libxo(3) in human and machine readable formats,
• output is explicitly indicated by the options,
• new option -D show all properties,
• update option -e specific sep as a separator,
• new option -F show flags,
• new option -I show internal nodes,
• new option -l show label,
• new option -r show tag-root with libxo,
• new option -m show format string,
• delete option -n simply do not use -N,
• updated option -N force to show name,
• new option -p show [property-name]: property-value,
• new option -S show magic nodes with -a,
• new option -V if "showable" display value, otherwise hide the state,
• new option -v force to show value,
• new option -y show id.

WWW: https://gitlab.com/alfix/nsysctl/

PR: 235870
Submitted by: Alfonso S. Siciliano <alfix86@gmail.com>
Reviewed by: mat
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D19469