Revert "local.c: avoid a NULL dereference on error."

This reverts commit 150f4c79e9f7143a57ec876a8cd98f3c2c172269 otherwise upstream commit https://github.com/jech/polipo/commit/6bb7900e512f2391a957801723d326c3ed3450a7
would need altering to the effect that it would not make much sense.
Revert "dns.c: check return value."

This reverts commit 44fd37caefadfbc95b25066d03b25e2047e20e0f which is now superceded by
upstream commit https://github.com/jech/polipo/commit/b0dc73a2547be7f29e0dead624f8720b718bc7f5
Revert "Fix a crash due to brittle va_lists on x86_64"

This reverts commit 3bec8400041cb6014eee7d243c04695a95d8c1e5 which is now superceded by
upstream commit https://github.com/jech/polipo/commit/d7cf597ffebde4e073b5ae005d53279681ea9258
Revert "server.c: Fixing unused variable."

This reverts commit 29f88dbf7b0731d7aba139625c9f5be17cea7352.

Needed because upstream commit https://github.com/jech/polipo/commit/cbec2ade2b96914c20598acda0434a7144c9ba18 removes the second part of this
and causes the upstream merge to fail.

The first part will be re-commited later on.
Fix atom leak in httpServerHandler.
Improve error checking in labelsToString.
Avoid warning in parseConfigFile.
Avoid spurious error messages in httpClientDiscardHandler.
Remove "Other HTTP Proxies" section from documentation.

Removes the "Other HTTP Proxies" section, which has been growing
increasingly obsolete.
Fix error handling in fillSpecialObject (forky version).
Avoid spurious error message in tunnels.
Remove compatibility definition of va_copy from log.c.

It's not needed here since it's being done in polipo.h.
Fix usage of va_copy.
poll(2) is defined in <poll.h>

<sys/poll.h> is deprecated, poll(2) is defined in <poll.h> in POSIX2.

This is needed for musl libc which does not provide <sys/poll.h>.