test: fix strtod() test

One strtod() test is broken since:

commit 8e211000025940b770794abf5754de61b4add0af
Author: Thomas Hindoe Paaboel Andersen <phomes@gmail.com>
Date:   Mon Aug 4 23:13:31 2014 +0200

    test: use fabs on doubles

The commit was right, so no reason to revert it, but the test was broken
before and only worked by coincidence. Convert "0,5" to "0.5" so we don't
depend on locales for double conversion (or well, we depend on "C" which
seems reasonable).
cgroup: never try to create files in cgroupfs, only open them for writing

This should have the benefit that cg_set_attribute() returns ENOENT
instead of EACCESS when we use it for non-existing attributes.
cgroup: downgrade log messages about non-existant cgroup attributes to LOG_DEBUG
sysctl: always write net.ipv4.conf.all.xyz= in addition to net.ipv4.conf.default.xyz=

Otherwise we have a boot-time race, where interfaces that popped up
after the sysctl service would get the settings applied, but all others
wouldn't.
networkd: rename UseDomainName to UseDomains

This option will also apply to the search domains, so make it plural.
networkctl: increase column width for link type to 18, to accomodate for 'ieee80211_radiotap'
unit: remove spurious newline
util: never use ether_ntoa(), since it formats with %x, not %02x, which makes ethernet addresses look funny
test: fix strtod test for real

The "0,5" syntax was actually right. The real problem is, the test should
only run if the local system has the de_DE.UTF-8 locale. Therefore, skip
the tests if setlocale() fails. This is kinda ugly, as it is done
silently, but we cannot skip partial tests with the current
infrastructure. Should be fine this way.