opts: more man pages
ROADMAP: initial commit of "curl the next few years"

To be further discussed, debated and edited
ROADMAP.md: make it markdown formatted
ROADMAP: removed, now ROADMAP.md
CURLOPT_URL: move up the text from "Notes"
opts: 37 additional man pages
opts: the final bunch of options as man pages

Now all current options have their own man pages.
opts: add the rest of the options

... and fixed mancheck to ignore obsolete options
opts: various corrections
opts: fixed some typos
opts: document CURLE_OUT_OF_MEMORY among other return values
CURLOPT_DNS_LOCAL_IP4.3: better short desc
curl_easy_setopt.3: shorten

shorten descriptions, mostly refer to the separate descriptions
opts: fixed mancheck for out-of-tree builds
curl_easy_setopt.3: refer to the individual man pages

With all the new individual option man pages created, this now refers to
each separate one instead of duplicaing the info. Also makes this page
easier to overview.
curl_easy_setopt.3: CURLOPT_POSTFIELDS is the exception

... to the always-copy-char *-argument.

And fix some minor mistakes.
lib: documentation updates in README.hostip

c-ares now does support IPv6;
avoid implying threaded resolver is Windows-only;
two referenced source files were renamed in 7de2f92
opts: the CURLOPT_SSL_ENABLE_*PN options are enabled by default
lib man pages: update easy setopt option references

... by using the "\fIopt(3)\fP" syntax they will be linked properly when
the web version of the page is generated.
curl_easy_setopt.3: fixed some typos