libpthread version bump

[Warner Losh]

> Having lived through the std{io,err,out} libc snafu, the
> libm.so.2 snafu, and now the libc.so.5/libpthread.so.1
> problem, I will once again suggest that *ALL* library
> version numbers should be bumped when a new branch is
> created.

Yes.  When a library version is bumped, *ALL* libraries that depend on
it need to be bumped.  Sadly, this has proven to be too difficult
because it would create an unholy maintanence problem for the ports.

Warner