[RHEL/6] Add remediations for requirements /dev/shm to be mounted with nodev, noexec, and nosuid mount options

This PR adds RHEL-6 specific [*] remediation fixes for:
* mount_option_dev_shm_nodev,
* mount_option_dev_shm_noexec, and
* mount_option_dev_shm_nosuid

rules. The three remediation fixes are analogous & are based on the following logic:
* load /etc/fstab's /dev/shm row content into local variable separating the 4-th field (the mount options) with the '#' character at the start and at the end
* split the aforementioned variable content into three variables (head, opts, & tail) according to the '#' character,
* further process the OPTS variable (mount options one):
 * replace 'defaults' keyword with the actual meaning (list of options) of that option at Red Hat Enterprise Linux 6 system
  * in each of nodev, noexec, nosuid particular cases replace any unprefixed 'no' version (e.g. dev, exec, suid) with the prefixed 'no' version (e.g. nodev, noexec, nosuid)
  * check the current OPTS variable content if it contains 'no' version for each of the dev, exec, & suid options on case-by-case basis (e.g. one concrete remediation fix would check just one option depending for which mount keyword / option it's intended for),
  * in case the 'no' version isn't there yet, append it
* finally reconstruct the final form of /dev/shm row (by concatenating original HEAD, modified OPTS & original TAIL variable values)
* and replace particular /etc/fstab's /dev/shm row with the newly constructed form.

[*] The need to be RHEL-6 specific wrt to the proposed form, because in RHEL-7 mounting of /dev/shm is handled by systemd in an automated way (based on udev discovery) so for RHEL-7 they will require to have different form.

Testing report:
--
The proposed change has been tested on RHEL-6 Server system (for all three of the enhanced options - dev, exec, & suid) & work's as expected.

Important note wrt to testing: Having the proposed change applied & running remediations for either one of the three will return 'error' state. But this is expected state - explanation:
* oscap when doing online remediation performs:
  * system scan against that property. If it fails, perform remediation (when requested) & after that runs scan of tha property again. If the subsequent (post-remediation) scan returns failure, oscap returns 'error' as exit status / message.

But since the OVAL partition / mount options check is based on checking of ```/proc/mounts``` file content (partition probe internally checks /proc/mounts content), since the system hasn't been rebooted from the moment remediation was applied & subsequent OVAL check was run, /proc/mounts still contains original options (IOW /dev/shm hasn't been remounted with the new setting from /etc/fstab), that post-remediation OVAL check with return failure & thus error.

Therefore to actually check the proposed remediation fixes are working properly it's necessary:
* either have a look at new /etc/fstab /dev/shm form after remediation (e.g. ensure that the row in question contains nodev, noexec, nosuid depending on which remediation was performed), or
* reboot the system to force /proc/mounts content to be regenerated (& therefore particular OVAL check in question to return truly real results).

Please review.

Thank you, Jan.