Ahaggar: add a null tree walker

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: use strlen() instead of sizeof()

The danger of copy/past, the code was running on x86_64 so we were
comparing 8 octets which can seem enough...

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: pass the call string to get_next_arg()

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: improve the get_arg_value() logic

Currently if we hit a const string then all chars that after a space
will be ignored, fix this by returning all the string.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: just return NULL if we hit the end of the call

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>