Merge branch 'mh/unify-xml-in-imap-send-and-http-push'

Update imap-send to reuse xml quoting code from http-push codepath,
clean up some code, and fix a small bug.

* mh/unify-xml-in-imap-send-and-http-push:
wrap_in_html(): process message in bulk rather than line-by-line
wrap_in_html(): use strbuf_addstr_xml_quoted()
imap-send: change msg_data from storing (ptr, len) to storing strbuf
imap-send: correctly report errors reading from stdin
imap-send: store all_msgs as a strbuf
lf_to_crlf(): NUL-terminate msg_data::data
xml_entities(): use function strbuf_addstr_xml_quoted()
Add new function strbuf_add_xml_quoted()

diff --cc strbuf.c
Simple merge

diff --cc strbuf.h
Simple merge