message: Add constructor with an int parameter for unix_fd
message: Add constructor with an int parameter for unix_fd

This allows implicit conversions from int to unix_fd. This is for
instance useful in functions where one wants to return an unix_fd by
just supplying the actual int file descriptor.

Change-Id: I118b8ec6b5a3d1533f5b05177127c28ad2539f3d
Signed-off-by: Waqar Hameed <d5c14662bf46e8e5d9de507114e98fd1658ebf51@axis.com>

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus