mosh: new package

Adding mosh, the mobile shell. Mosh uses ssh or dropbear as an initial
transport to start mosh-server which uses UDP to communicate with the
client.  Supports a predictive model to enhance performance on weak
connections and compensate for general lag. Also supports
disconnections and reconnections seamlessly.

[Thomas:
  - Add missing dependency on host-pkgconf, as noticed by Yann
    E. Morin.
  - Indicate that openssh/dropbear is a runtime dependency.
  - Pass some variables in the configure environment to tell that SSP
    support is not available when it isn't. Otherwise, it misdetects
    the SSP support as being available, causing a build failure.]

Signed-off-by: Christian Stewart <christian@paral.in>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>