src: fix `fw-admin -c' calls

Previous to this patch, `fw-admin -c' calls were failing:

I: Checking variables /etc/fw-admin.d/rules/
grep: /etc/fw-admin.d/rules/: Es un directorio
grep: /etc/fw-admin.d/rules/: Es un directorio
grep: /etc/fw-admin.d/rules/: Es un directorio
I: Checking iptables syntax /etc/fw-admin.d/rules/
cat: /etc/fw-admin.d/rules/: Es un directorio
I: Checking ip6tables syntax /etc/fw-admin.d/rules/
cat: /etc/fw-admin.d/rules/: Es un directorio
[...]

With this patch, a the call is fixed, and while at it, a new way of operations
are implemented:

* To check all files in $START_ORDER:
 # fw-admin -c
* To check all files in the cmd arguments:
 # fw-admin -c file1 file2 file3
* Or check just one concrete file:
 # fw-admin -c file

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>