portmap: improve installation of binaries

This commit explicits the permissions with which executables are
installed, and also makes sure that the build is aborted if one of the
binaries cannot be installed, instead of silently ignoring the error.

[Peter: fixop typos in commit message as noted by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>