spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

libalias: Fix splay comparsion bug

Comparing elements in a tree requires transitiviy. If a < b and b < c
then a must be smaller than c. This way the tree elements are always
pairwise comparable.

Tristate comparsion functions returning values lower, equal, or
greater than zero, are usually implemented by a simple subtraction of
the operands. If the size of the operands are equal to the size of
the result, integer modular arithmetics kick in and violates the
transitivity.

Example:
Working on byte with 0, 120, and 240. Now computing the differences:
120 - 0 = 120
240 - 120 = 120
240 - 0 = -16

MFC after: 3 days

libalias: Fix compile time warning about unused functions

Compiling libalias results in warnings about unused functions.
Those warnings are caused by clang's heuristic to consider an inline
function as in use, iff the declaration is in a *.c file.
Declarations in *.h files do not emit those warnings.

Hence the declarations must be moved to an extra *.h file.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D30844