Fixes#7629.
nfs-server.service seems to load kernel modules it needs itself, while nfsd appears in `/proc/filesystems` only after the kernel module has been loaded, so vagrant fails to detect NFS server until it's started first time after the system has booted. This checks if the NFS service actually exists and hopes that it'll figure the stuff out itself.
`list-unit-files` is utilized rather than `list-units` because systemd seems to not list units that are disabled: https://lists.fedoraproject.org/pipermail/devel/2011-November/159117.html
Currently the code tries to detect if Arch uses systemd via checking
comm= of PID 1. As access to proc filesystem might be restricted due to
hideproc set and systemd is default for all Arch Linux installations
since October 2012, let's just ditch that check.
Call it with "--no-pager"
Without it, if the user has a pager (more/less/etc.) configured and
call vagrant up with NFS shares, systemctl would use the pager, and
that would probably require an unnecessary key press from the user.
Alexander Kurilo
Bartłomiej Piotrowski
penz
Mitchell Hashimoto