getlocalbase: Make default path actually configurable

We include paths.h in getlocalbase.c, which defines _PATH_LOCALBASE.
This will override a definition on the command line, meaning it is
impossible to override like how the manpage says you can, and it means
the code to provide a fallback default is dead as the macro is always
defined.

Instead, introduce a new LOCALBASE_PATH macro like LOCALBASE_CTL_LEN
that can be set on the command line and will default to the system's
existing _PATH_LOCALBASE to avoid duplicating the default here.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40975