netsnmp: Make server and client installation optional

The netsnmp package consists of both a server and a number of client
tools, this makes the installation of both parts optional.

[Thomas:
  - Add explicit --enable-{agent,applications}, and use positive logic.
  - Remove "net-snmp" from the new Config.in option prompts, since
    they are already visible under the netsnmp package option.]

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>