imap-send: drop global `imap_server_conf` variable

imap-send: drop global `imap_server_conf` variable

In "imap-send.c", we have a global `sturct imap_server_conf` variable
that keeps track of the configuration of the IMAP server. This variable
is being populated mostly via the Git configuration.

Refactor the code to allocate the structure on the stack instead of
having it globally. This change allows us to track its lifetime more
closely.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>