Change FD_SETSIZE size to unsigned due to FreeBSD

In FreeBSD, FD_SETSIZE is defined as follow:
This U means unsigned and isn't there on Linux. Therefore, this commit
adds a test to check if FD_SETSIZE is below 0 - in which case it's an
error - and then make the comparison unsigned for it to work on every
OS.
Simplify package installation on FreeBSD

Regroup three `pkg install' commands into one.