strlcat: remove function

This function was only used twice, both in places where performance
isn't crucial (socks + if2ip). Removing the use of this function removes
the need to have our private version for systems without it == reduced
amount of code.

Also, in the SOCKS case it is clearly better to fail gracefully rather
than to truncate the results.

This work was triggered by a bug report on the strcal prototype in
strequal.h.

strlcat was added in commit db70cd28 in February 2001!

Bug: http://curl.haxx.se/bug/view.cgi?id=1192
Reported by: Jeremy Huddleston