MidnightBSD may also not have strnlen

Provide an implementation for ```strnlen``` on MidnightBSD (a fork/clone of FreeBSD).

I've been seeing these in the perl bindings test reports:
```
Can't load '/usr/home/cpan/pit/bare/conf/perl-5.14.0/.cpanplus/5.14.0/build/Git-Raw-0.35/blib/arch/auto/Git/Raw/Raw.so' for module Git::Raw: /usr/home/cpan/pit/bare/conf/perl-5.14.0/.cpanplus/5.14.0/build/Git-Raw-0.35/blib/arch/auto/Git/Raw/Raw.so: Undefined symbol "strnlen" at /usr/home/cpan/pit/bare/perl-5.14.0/lib/5.14.0/amd64-midnightbsd/DynaLoader.pm line 190.
 at /usr/home/cpan/pit/bare/conf/perl-5.14.0/.cpanplus/5.14.0/build/Git-Raw-0.35/blib/lib/Git/Raw/Blame.pm line 6
```