xmlCtxtReadFile doesn't work with literal IPv6 URLs

https://bugzilla.gnome.org/show_bug.cgi?id=694185

RedHat Bug 624626 discusses the new behavior of libxml regarding brackets
around IPv6 addresses.  In earlier versions such as 2.6.27, uri.c stripped the
brackets (e.g. uri->server == "fdf2:1e39:73d1:934e::119"); in the current
version it returns IPv6 addresses with brackets intact (e.g. uri->server
== "[fdf2:1e39:73d1:934e::119]").

Thus in 2.9.0, xmlCtxtReadFile() has a problem when it is passed a URL
containing a literal IPv6 address.  xmlCtxReadFile() and its subroutines pass
uri->server unchanged to getaddrinfo(), which doesn't recognize a bracketed
IPv6 address, so the read fails.

This strips the [ and ] from IPv6 addresses allowing getaddrinfo()
to work properly with such URIs.