Make use of the getlocalbase() function for run-time adjustment of the
local software base directory, as committed in SVN rev. 367813.

The pkg and mailwrapper programs used the LOCALBASE environment variable
for this purpose and this functionality is preserved by getlocalbase().

After this change, the value of the user.localbase sysctl variable is used
if present (and not overridden in the environment).

The nvmecontrol program gains support of a dynamic path to its plugin
directory with this update.

Differential Revision: https://reviews.freebsd.org/D27237