Fixes to enable building on FreeBSD

[esell]

Hello, first let me say that none of this work was done by me, I just applied the patches and created a git repo for the results :).

I have applied all of the patches from https://svnweb.freebsd.org/ports/head/net-mgmt/seafile-server/files/ which allow seafile-server to build cleanly on FreeBSD. I added one tweak with regards to P_KTHREAD as well. I have tested this branch on Ubuntu as well and it appears to build cleanly with the changes.