Bug #671172 - camel: smtp: Make EHLO keywords check case insensitive

Bug #671172 - camel: smtp: Make EHLO keywords check case insensitive

According to section 4.1.1.1 of RFC 2821 (last paragraph,
see tools.ietf.org/html/rfc2821#section-4.1.1.1):

   Although EHLO keywords may be specified in upper, lower, or mixed
   case, they MUST always be recognized and processed in a case-
   insensitive manner.  This is simply an extension of practices
   specified in RFC 821 and section 2.4.1.

Current smtp_helo() implementation is using strncmp() - replace them
with g_ascii_strncasecmp().