r308996 broke IP literals by assuming that a colon could only occur as
a separator between host and port, and using strchr() to search for it.
Rewrite fetch_resolve() so it handles bracketed literals correctly, and
remove similar code elsewhere to avoid passing unbracketed literals to
fetch_resolve(). Remove #ifdef INET6 so we still parse IP literals
correctly even if we do not have the ability to connect to them.

While there, fix an off-by-one error which caused HTTP 400 errors to be
misinterpreted as redirects.

PR: 217723
MFC after: 1 week
Reported by: bapt, bz, cem, ngie