Description of problem:
Systemd allows to join multiple cgroup controllers into one item (see `man systemd.conf`, JoinControllers). Default value for this option is:

JoinControllers=cpu,cpuacct

which results to this line in /etc/mtab:

cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpuacct,cpu 0 0

Lens file for /etc/fstab and /etc/mtab forbids having comma in spec definition. So /etc/mtab is not parsed correctly and can't be used by augeas.

Version-Release number of selected component (if applicable):
augeas-0.9.0-1.fc16.x86_64
systemd-37-2.fc16.x86_64

Steps to Reproduce:
1. run `augtool`
2. print /augeas/files/etc/mtab/error/
  
Actual results:
/augeas/files/etc/mtab/error = "parse_failed"
/augeas/files/etc/mtab/error/pos = "936"
/augeas/files/etc/mtab/error/line = "14"
/augeas/files/etc/mtab/error/char = "0"
/augeas/files/etc/mtab/error/lens = "/usr/share/augeas/lenses/dist/fstab.aug:32.12-.42:"
/augeas/files/etc/mtab/error/message = "Iterated lens matched less than it should"


Expected results:
No error