Replace strdup() calls for string_alloc() calls

Replace strdup() calls for string_alloc() calls

As reported by Bill Parker in trac #600, strdup() return values are not
always correctly checked for failed allocations.  This patch adds missing
checks by using string_alloc(), which performs the required checks.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <561130FC.8090008@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10176
Signed-off-by: Gert Doering <gert@greenie.muc.de>