Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Fixes segmentation fault.

Don't use va_list.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

This caused random data to be send via icy-server if the played
song had no tags.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

icy_server_metadata_string() is only called by
icy_server_metadata_page().

[mk: folded with patch "Put icy related functions in extra source
files"; moved icy_server.c from HAVE_CURL to ENABLE_HTTPD_OUTPUT;
removed an unused variable]