manager: put bin before sbin for user instances

Traditionally, user logins had a $PATH in which /bin was before /sbin, while
root logins had a $PATH with /sbin first. This allows the tricks that
consolehelper is doing to work. But even if we ignore consolehelper, having the
path in this order might have been used by admins for other purposes, and
keeping the order in user sessions will make it easier the adoption of systemd
user sessions a bit easier.

Fixes #733.
https://bugzilla.redhat.com/show_bug.cgi?id=1744059

OOM handling in manager_default_environment wasn't really correct.
Now the (theorertical) malloc failure in strv_new() is handled.

Please note that this has no effect on:
- systems with merged /bin-/sbin (e.g. arch)

- when there are no binaries that differ between the two locations.

  E.g. on my F30 laptop there is exactly one program that is affected:
  /usr/bin/setup -> consolehelper.

  There is less and less stuff that relies on consolehelper, but there's still
  some.

So for "clean" systems this makes no difference, but helps with legacy setups.

$ dnf repoquery --releasever=31 --qf %{name} --whatrequires usermode
anaconda-live
audit-viewer
beesu
chkrootkit
driftnet
drobo-utils-gui
hddtemp
mate-system-log
mock
pure-ftpd
setuptool
subscription-manager
system-config-httpd
system-config-rootpassword
system-switch-java
system-switch-mail
usermode-gtk
vpnc-consoleuser
wifi-radar
xawtv