BUG/MINOR: fd: allow fd_write_frag_line() to run before fdtab allocation

BUG/MINOR: fd: allow fd_write_frag_line() to run before fdtab allocation

With traces enabled on the command line using "-dt ssl:developer:clean",
a trace event is emitted during the configuration check if an SSL bind
line uses "generate-certificates".

check_config_validity() calls ->prepare_bind_conf() for every bind line.
For SSL, this is ssl_sock_prepare_bind_conf(). When "generate-certificates"
is set, that function checks that a default certificate exists, and calls
ssl_sock_choose_sni_ctx() for this. This happens long before
init_pollers() is called, so fdtab is still NULL. fd_write_frag_line()
reads fdtab[fd].state to take the per-fd lock and to test FD_INITIALIZED,
and the process segfaults as soon as the trace is written to stderr.

fd_write_frag_line() has read fdtab since 931d8b79a (v2.1), but no trace
could be emitted that early at that time. The crash became possible with
105c1ca13 (v3.2-dev13), which added traces to ssl_sock_choose_sni_ctx();
ssl_sock_prepare_bind_conf() has called this function from the
configuration check since b80635a7e (v3.0).

At this stage the process is still single-threaded, so the per-fd lock is
not needed. Skip the fdtab accesses when fdtab is not allocated yet, and
call writev() directly.

This should be backported to 3.2 and newer.