Add support of /etc/sysctl.d/*.conf in the sysctl parameter check template for RHEL6

[ssato]

This should fix the issue #36, I guess.

BTW, I do not regenerate actual checks in RHEL/6/input/checks/sysctl_* yet.
If I need to do that please let me know.

[ssato]

The loading order of sysctl conf files are as follows.

a) /etc/sysctl.conf, /etc/sysctl.d/.conf in RHEL 6. See also: apply_sysctl() in /etc/rc.d/init.d/functions.
b) /usr/lib/sysctl.d/.conf, /run/sysctl.d/.conf, /etc/sysctl.d/.conf; /etc/sysctl.d/99-sysctl.conf is symlinked to /etc/sysctl.conf in RHEL7/F20+. See also sysctl.d(5).

I don't know how to write it to take care of the ordering of priorities of these
sysctl configuration files. However, the criteria of sysctl consists of a couple of
criterion, both live (sysctl command) and static (/etc/sysctl.conf and /etc/sysctl.d/*.conf)
sysctl parameters must be set (//criteria[@operator = 'AND']). That is, it implies
that target parameter must be set in the highest priority of static configuration files
anyway. So I guess this change is enough for most cases.

[ssato]

If it's OK, could you please merge it ? After that I'll make another PR to merge
changes of generated files if you don't mind.

[shawndwells]

@ssato For commit e7af034 / the RHEL 7 check, update the platform tag and I'll merge.

[shawndwells]

@ssato went ahead and merged. manually updated the platform tag.