Symlink fixup

[nigels-com]

It appears that PySquashfsImage doesn't currently support symlinks.

This change adds some support by ensuring that read_inode happens for both files and links.

To test this I used the current slack snap via: https://snapcraft.io/slack

Before:

$ python2 PySquashfsImage/PySquashfsImage.py slack_14.snap /lib/x86_64-linux-gnu
--------------/lib/x86_64-linux-gnu                              --------------
FOLDER /lib/x86_64-linux-gnu
	libbsd.so.0          <dir>
	libbsd.so.0.8.2      84976
	libdbus-1.so.3       84976
	libdbus-1.so.3.14.6  327216
	libexpat.so.1        327216
	libexpat.so.1.6.0    171056
	libglib-2.0.so.0     171056
	libglib-2.0.so.0.4800.2 1154768
	libjson-c.so.2       1154768
	libjson-c.so.2.0.0   48504
	libkeyutils.so.1     48504
	libkeyutils.so.1.5   18432
	libpcre.so.3         18432
	libpcre.so.3.13.2    460760
	libpng12.so.0        460760
	libpng12.so.0.54.0   156960
	libwrap.so.0         156960
	libwrap.so.0.7.6     39360

After:

$ python2 PySquashfsImage/PySquashfsImage.py slack_14.snap /lib/x86_64-linux-gnu
--------------/lib/x86_64-linux-gnu                              --------------
FOLDER /lib/x86_64-linux-gnu
	libbsd.so.0          ->
	libbsd.so.0.8.2      84976
	libdbus-1.so.3       ->
	libdbus-1.so.3.14.6  327216
	libexpat.so.1        ->
	libexpat.so.1.6.0    171056
	libglib-2.0.so.0     ->
	libglib-2.0.so.0.4800.2 1154768
	libjson-c.so.2       ->
	libjson-c.so.2.0.0   48504
	libkeyutils.so.1     ->
	libkeyutils.so.1.5   18432
	libpcre.so.3         ->
	libpcre.so.3.13.2    460760
	libpng12.so.0        ->
	libpng12.so.0.54.0   156960
	libwrap.so.0         ->
	libwrap.so.0.7.6     39360

And w.r.t the mounted snap on my filesystem:

$ ls -lah /snap/slack/14/lib/x86_64-linux-gnu/
total 2.4M
drwxr-xr-x 2 root root  441 May 22 12:59 .
drwxr-xr-x 3 root root   39 Sep 18  2018 ..
lrwxrwxrwx 1 root root   15 Jan 28  2016 libbsd.so.0 -> libbsd.so.0.8.2
-rw-r--r-- 1 root root  83K May 22 12:59 libbsd.so.0.8.2
lrwxrwxrwx 1 root root   19 Jan 13  2017 libdbus-1.so.3 -> libdbus-1.so.3.14.6
-rw-r--r-- 1 root root 320K May 22 12:59 libdbus-1.so.3.14.6
lrwxrwxrwx 1 root root   17 Jun 27  2017 libexpat.so.1 -> libexpat.so.1.6.0
-rw-r--r-- 1 root root 168K May 22 12:59 libexpat.so.1.6.0
lrwxrwxrwx 1 root root   23 Sep 18  2018 libglib-2.0.so.0 -> libglib-2.0.so.0.4800.2
-rw-r--r-- 1 root root 1.2M May 22 12:59 libglib-2.0.so.0.4800.2
lrwxrwxrwx 1 root root   18 Jan 13  2015 libjson-c.so.2 -> libjson-c.so.2.0.0
-rw-r--r-- 1 root root  48K May 22 12:59 libjson-c.so.2.0.0
lrwxrwxrwx 1 root root   18 Dec 11  2015 libkeyutils.so.1 -> libkeyutils.so.1.5
-rw-r--r-- 1 root root  18K May 22 12:59 libkeyutils.so.1.5
lrwxrwxrwx 1 root root   17 Mar 25  2016 libpcre.so.3 -> libpcre.so.3.13.2
-rw-r--r-- 1 root root 450K May 22 12:59 libpcre.so.3.13.2
lrwxrwxrwx 1 root root   18 Jul 11  2018 libpng12.so.0 -> libpng12.so.0.54.0
-rw-r--r-- 1 root root 154K May 22 12:59 libpng12.so.0.54.0
lrwxrwxrwx 1 root root   16 Jan 13  2014 libwrap.so.0 -> libwrap.so.0.7.6
-rw-r--r-- 1 root root  39K May 22 12:59 libwrap.so.0.7.6

[nigels-com]

With getLink:

$ python2 PySquashfsImage/PySquashfsImage.py slack_14.snap /lib/x86_64-linux-gnu
--------------/lib/x86_64-linux-gnu                              --------------
FOLDER /lib/x86_64-linux-gnu
	libbsd.so.0 -> libbsd.so.0.8.2
	libbsd.so.0.8.2                                                 84976
	libdbus-1.so.3 -> libdbus-1.so.3.14.6
	libdbus-1.so.3.14.6                                            327216
	libexpat.so.1 -> libexpat.so.1.6.0
	libexpat.so.1.6.0                                              171056
	libglib-2.0.so.0 -> libglib-2.0.so.0.4800.2
	libglib-2.0.so.0.4800.2                                       1154768
	libjson-c.so.2 -> libjson-c.so.2.0.0
	libjson-c.so.2.0.0                                              48504
	libkeyutils.so.1 -> libkeyutils.so.1.5
	libkeyutils.so.1.5                                              18432
	libpcre.so.3 -> libpcre.so.3.13.2
	libpcre.so.3.13.2                                              460760
	libpng12.so.0 -> libpng12.so.0.54.0
	libpng12.so.0.54.0                                             156960
	libwrap.so.0 -> libwrap.so.0.7.6
	libwrap.so.0.7.6                                                39360