daemon: use strbuf for hostname info

daemon: use strbuf for hostname info

Convert hostname, canon_hostname, ip_address and tcp_port to strbuf.
This allows to get rid of the helpers strbuf_addstr_or_null() and STRARG
because a strbuf always represents a valid (initially empty) string.

sanitize_client() is not needed anymore and sanitize_client_strbuf()
takes its place and name.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>