Revert "chore, don't check libev availability during configure"

This reverts commit 0493037e142fdfda60e242a6bb04be5bbd36fe25.
chore, bring back ud-sock helpers
minor, restart work on httpd library, libbeef, based on libev
minor, provide setsock_sndtimeo()/setsock_rcvtimeo() (properly)

Also, properly declare timeout parameter as unsigned int, timeouts are in microseconds.
chore, turn timeout slot into unsigned, ...

also document the unit.
minor, provide connection pooling and actual data callback in libbeef httpd
chore, modernise nifty.h
hygiene, include stdlib.h in gand-cfg.c for getenv()
minor, snarf HTTP verbs with gperf
minor, actually parse the http request and fill in basic request struct

The request object carries only the essentials:
- the HTTP verb
- the host the request is sent to
- the resource path
- and the query

Further and custom inspection of the headers is possible via the
HDR slot in the request.
minor, provide helper routines to obtain http header values and query fields
fix, PEBKAC, be extra careful with the pointers whilst parsing the headers
minor, \nul out the second CRLF pair of a header boundary

So API users can split by lines.
minor, provide a rich struct for httpd responses

The worker function is asked to fill in a gand_httpd_res_t object.
minor, support www_dir, change there to serve static files
minor, clarify failures during httpd set up
minor, allow to specify the server string upon make_httpd()
chore, check for sendfile.h
minor, provide transmission queue and send off static data
minor, operate in terms of the connection and write-queue objects in callbacks