Update the main switch in get() to match sysctl(8) implementation.

Previously, both sysctl(8) and lua-sysctl were using the oidfmt()
function as reference to determine the variable's type. sysctl(8) is
now using only the CTLTYPE and this commit reflect theses
modifications.

see also https://github.com/freebsd/freebsd/commit/6648b8cede12bffb36e7260cfcf022cf2315acf1