ifupdown2: syntax-check: exit with 1 if syntax errors

Ticket: CM-7995
Reviewed By: CCR-3850
Testing Done: Tested exit code on syntax errors

This patch adds members 'errors' and 'warns' to networkinterfaces.py
to track errors and warns during parsing interfaces file.

This patch also adds --syntax-check option to ifreload
given people seem to use ifreload more than ifup these days.

$ ifreload --syntax-check -a
error: /etc/network/interfaces: iface swp1.200: unsupported keyword (roopa-attr)
$ echo $?
1

(cherry picked from commit e643a136fcf5d387ff0f9a31cb6a6af4983e1012)

man.rst/ifreload.8.rst

@@ -14,7 +14,7 @@ reload network interface configuration
 
 SYNOPSIS
 ========
-    ifreload [-h] (-a|-c) [-v] [-d] [-f] [-n] 
+    ifreload [-h] (-a|-c) [-v] [-d] [-f] [-n] [-s]
 
 DESCRIPTION
 ===========
@@ -61,6 +61,8 @@ OPTIONS
                           then each dependent interface must be specified in order
                           to be excluded.
 
+    -s, --syntax-check    Only run the interfaces file parser
+
 
 EXAMPLES
 ========

man.rst/ifup.8.rst

@@ -22,7 +22,7 @@ SYNOPSIS
 ========
 
     ifup [-h] [-a] [-v] [-d] [--allow CLASS] [--with-depends]
-       **[-X EXCLUDEPATS] [-f] [-n] [--print-dependency {list,dot}]**
+       **[-X EXCLUDEPATS] [-f] [-n] [-s] [--print-dependency {list,dot}]**
        **[IFACE [IFACE ...]]**
 
     ifdown [-h] [-a] [-v] [-d] [--allow CLASS] [--with-depends]
@@ -115,6 +115,8 @@ OPTIONS
                           your state file is corrupted or you want down to use
                           the latest from the interfaces file
 
+    -s, --syntax-check    Only run the interfaces file parser
+
 EXAMPLES
 ========
     # bringing up all interfaces