Updated terminology in docs, comments and phrases to refer to C strings
as "null-terminated". Done to unify with how most other C oriented docs
refer of them and what users in general seem to prefer (based on a
single highly unscientific poll on twitter).

Reported-by: coinhubs on github
Fixes #5598
Closes #5608

.. because checksrc's copyright year check stopped working.

Ref: https://github.com/curl/curl/pull/4547

Closes https://github.com/curl/curl/pull/4549

This fixes potential out-of-buffer access on "file:./" URL

$ valgrind curl "file:./"
==24516== Memcheck, a memory error detector
==24516== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==24516== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==24516== Command: /home/even/install-curl-git/bin/curl file:./
==24516==
==24516== Conditional jump or move depends on uninitialised value(s)
==24516==    at 0x4C31F9C: strcmp (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==24516==    by 0x4EBB315: seturl (urlapi.c:801)
==24516==    by 0x4EBB568: parseurl (urlapi.c:861)
==24516==    by 0x4EBC509: curl_url_set (urlapi.c:1199)
==24516==    by 0x4E644C6: parseurlandfillconn (url.c:2044)
==24516==    by 0x4E67AEF: create_conn (url.c:3613)
==24516==    by 0x4E68A4F: Curl_connect (url.c:4119)
==24516==    by 0x4E7F0A4: multi_runsingle (multi.c:1440)
==24516==    by 0x4E808E5: curl_multi_perform (multi.c:2173)
==24516==    by 0x4E7558C: easy_transfer (easy.c:686)
==24516==    by 0x4E75801: easy_perform (easy.c:779)
==24516==    by 0x4E75868: curl_easy_perform (easy.c:798)

Was originally spotted by
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10637
Credit to OSS-Fuzz

Closes #3039

Found when updating test 1395, which I did to increase test coverage of
this source file...

Closes #1535

Follow up to commit 7db9782dd6.

It isn't used by the code in current conditions but for safety it seems
sensible to at least not crash on such input.

Extended unit test 1395 to verify this too as well as a plain "/" input.