MINOR: haload: add -get/-head/-post options to select the HTTP method

MINOR: haload: add -get/-head/-post options to select the HTTP method

Only GET (or HEAD globally via -I) could be sent before. Add -get,
-head and -post <size> as positional options, like -h(0|1|2|2c|3), to
pick the method and, for POST, the body size, per URL.

The method and body size are stored per URL path (struct hld_path) so
distinct paths sharing the same server address keep their own
settings. The POST body is filled from a static buffer, streamed
across several output buffers for large sizes, with a matching
Content-Length header.

Remove -I, superseded by -head. Update doc/haload.txt.