imap-send: cleanup execl() call to use NULL sentinel instead of 0

imap-send: cleanup execl() call to use NULL sentinel instead of 0

Some versions of gcc check that calls to the exec() family have the proper
sentinel for variadic calls. This should be (char *) NULL according to the
man page. Although for all other purposes the 0 is equivalent, gcc
nevertheless does emit a warning for 0 and not for NULL. This also makes the
usage consistent throughout git.

The whitespace in function calls throughout imap-send.c has its own style,
so I left it that way.

Signed-off-by: Junio C Hamano <junkio@cox.net>