6in4: detect curl and busybox wget

b52053b9 6in4: https support for he.net tunnel api
introduced HTTPS support using wget.
The busybox version of wget, however, doesn't support the -V option,
thus poluting logfiles with a full invalid-parameter-output.
Redirect stderr to fix that.
As libcurl and curl support selecting the SSL library of your choice,
also add support for curl which is more commonly used on OpenWrt than
"real" wget which needs libopenssl.
Also make sure to respect SSL_CERT_DIR and increase timeouts.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43228 3c298f89-4303-0410-b956-a3cf2f4a3e73