fixed compilation warning against gcc-4.4

fixed compilation warning against gcc-4.4

* Change arguments of tun_gifindex from:
int tun_gifindex(struct tun_t *this, int *index)
to
int tun_gifindex(struct tun_t *this, __u32 *index)
solves:
../lib/tun.c: In function ‘tun_addaddr’:
../lib/tun.c:265: warning: pointer targets in passing argument 2 of
‘tun_gifindex’ differ in signedness
../lib/tun.c:88: note: expected ‘int *’ but argument is of type ‘__u32
*’

* handle system return code and returns -1 if system failed
solves:
../lib/tun.c: In function ‘tun_runscript’:
../lib/tun.c:895: warning: ignoring return value of ‘system’, declared
with attribute warn_unused_result

* do not dereference a pointer cast and use it as an lvalue
see
http://www.mail-archive.com/svn-commits@lists.digium.com/msg50931.html
solves:
../lib/tun.c: In function ‘tun_route’:
../lib/tun.c:533: warning: dereferencing pointer ‘({anonymous})’ does
break strict-aliasing rules
../lib/tun.c:533: note: initialized from here
../lib/tun.c:534: warning: dereferencing pointer ‘({anonymous})’ does
break strict-aliasing rules
../lib/tun.c:534: note: initialized from here
../lib/tun.c:535: warning: dereferencing pointer ‘({anonymous})’ does
break strict-aliasing rules
../lib/tun.c:535: note: initialized from here
../lib/tun.c: In function ‘tun_setaddr’:
../lib/tun.c:435: warning: dereferencing pointer ‘({anonymous})’ does
break strict-aliasing rules
../lib/tun.c:435: note: initialized from here
../lib/tun.c:452: warning: dereferencing pointer ‘({anonymous})’ does
break strict-aliasing rules
../lib/tun.c:452: note: initialized from here
../lib/tun.c:465: warning: dereferencing pointer ‘({anonymous})’ does
break strict-aliasing rules
../lib/tun.c:465: note: initialized from here

Signed-off-by: Emmanuel Bretelle <chantra@debuntu.org>