Update license to strptime(3) implementation.

Our strptime(3) implementation was the base for the illumos
implementation and after contacting the author, Kevin Rudy
stated the code is under a 2-Clause BSD License [1]

After reviewing our local changes to the file in question,
the FreeBSD Foundation has agreed that their contributions
to this file are not required to carry clause 3 or 4 so
the file can be relicensed as in Illumos [2].

References:

[1] https://www.illumos.org/issues/357
[2] Illumos Revision: 13222:02526851ba75

Approved: core (jhb)
Approved: FreeBSD Foundation (emaste)
MFC after: 4 days