yhttpd: port ychat fixes, get it building + stable in Docker (task 9s0)

Ports the ychat revival fixes (unsigned/size_t npos truncation, ofstream
== NULL, tool::trim OOB, CGI popen -> execve, www.yChat.org links, g++
version gate, config.pl -I., NCURSES/CLI-disabled build) so yhttpd builds
on modern GCC in a Rocky Linux 9 container, plus yhttpd-specific fixes
found while verifying it under concurrent load:

- listen() backlog was hardcoded to 1; bumped to SOMAXCONN.
- sock::_close() closed sockets with unread request bytes still in the
  kernel receive buffer (read_http() only reads the GET line), so Linux
  sent an abortive RST instead of a FIN, racing the client's read of the
  response ("connection reset by peer" even though it was delivered).
  Fixed with a non-blocking, bounded drain before close() - confirmed via
  tcpdump: RSTs on every response before, zero after, across 140+
  requests / concurrent bursts of 20.
- Removed a duplicate _make_server_socket() call in start() (wrap.cpp's
  init_wrapper() already makes it before start() runs) that leaked a fd
  and would have double-initialized SSL if OPENSSL is ever enabled;
  caught by fresh-context review, documented honestly in DOCKER.md.
- src/configure's dependency-checker predates 64-bit multilib distros
  (only checked /usr/lib, never /usr/lib64) and was missing an ncur
  move-aside entry for the NCURSES-disabled build.

Added Dockerfile/.dockerignore/DOCKER.md documenting the build, the fixes,
and the one known-but-unfixed landmine (a SIGILL heap corruption in
sock::_close that reproduces on newer host GCC/glibc but not in the
container - latent, not fixed).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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