Add newRepurpose the useless Tn macro .Slkey define to be used for documenting sysctls to mandoc.storing
man(1) will use apropos(1) internally to search for sysctl namesthe pseudo _tun macro when .Va is found inside of the "LOADER TUNABLES"
as a last resort option.or "SYSCTL VARIABLES" sections.
Teach man(1) to do apropos search with _tun as key if we did not match
using the current logic.