api/http: Make some stylistic changes

   o  No semicolons after blocks

   o  No spaces at end of lines

   o  Indentation with tabs

   o  Declaration of HTTP function pointers as static
api/http: Use connection userdata instead of list of clients
api/http: smalloc() uses calloc() which initializes memory to 0 for us
api/http: Allocate some buffers on the stack, instead of with smalloc
api/http: Allocate query_string differently

Also provide query_string_concat and query_string_reset to make
modification of the query_string easier.