Mail: fixed backslash handling in IMAP literals. Previously, s->backslash was set if any of the arguments was a quoted string with a backslash character. After successful command parsing this resulted in all arguments being filtered to remove backslashes. This is, however, incorrect, as backslashes should not be removed from IMAP literals. For example: S: * OK IMAP4 ready C: a01 login {9} S: + OK C: user\name "pass\"word" S: * BAD internal server error resulted in "Auth-User: username" instead of "Auth-User: user\name" as it should. Fix is to apply backslash filtering on per-argument basis during parsing.