Make --http-keepalive require an argument.

[funkybob]

This flag accepts a timeout value, but is typically mistaken for a bool.
Update the description to help clarify, and require the option value.

[xrmx]

Uhm it looks like there's places where it is used as boolean because uwsgi_opt_set_int will set it a 1 if not value is passed.

plugins/http/http.c:                        if (uhttp.keepalive && !uwsgi_strncmp("HTTP/1.1", 8, base, ptr-base)) {
plugins/http/http.c:                    if (uhttp.keepalive && !uwsgi_strncmp("HTTP/1.1", 8, base, ptr-base)) {
plugins/http/http.c:                    if (uhttp.keepalive > 1) {
plugins/http/http.c:                            http_set_timeout(peer->session->main_peer, uhttp.keepalive);

[funkybob]

So perhaps I should drop this, and just work on improving the docs?

BTW - what unit is that timeout in? I assumed seconds, but, well...

[karmux]

Merge?
Boolean checks should work as expected after this change.
In another PR they can be changed to if (uhttp.keepalive > 0).
Related to #1245.