example: add smtp-multi.c

An example application source code sending SMTP mail with the multi
interface. It is based on the code Alona Rossen provided, which in turn
is based on existing example/test code, and I converted it even more
into a decent example with a fair multi API use, put the info required
to edit at the top and I added some comments.

docs/examples/Makefile.inc

@@ -3,7 +3,8 @@ check_PROGRAMS = 10-at-a-time anyauthput cookie_interface debug fileupload \
   fopen ftpget ftpgetresp ftpupload getinfo getinmemory http-post httpput  \
   https multi-app multi-debugcallback multi-double multi-post multi-single \
   persistant post-callback postit2 sepheaders simple simplepost simplessl  \
-  sendrecv httpcustomheader certinfo chkspeed ftpgetinfo ftp-wildcard
+  sendrecv httpcustomheader certinfo chkspeed ftpgetinfo ftp-wildcard \
+  smtp-multi.c
 
 # These examples require external dependencies that may not be commonly
 # available on POSIX systems, so don't bother attempting to compile them here.