execute_command_get_output fails with /sbin/ip

/sbin/ip seems to interact badly with the stdout redirection
mechanism used by forkexecd (where output is redirected to deleted
files). Work around this by using a pipe instead.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #8 from djs55/master

Fix interaction with /sbin/ip and execute_command_get_output