Mail: smtp pipelining support. Basically, this does the following two changes (and corresponding modifications of related code): 1. Does not reset session buffer unless it's reached it's end, and always wait for LF to terminate command (even if we detected invalid command). 2. Record command name to make it available for handlers (since now we can't assume that command starts from s->buffer->start).

--- a/src/mail/ngx_mail.h
+++ b/src/mail/ngx_mail.h
@@ -234,6 +234,8 @@ typedef struct {
     ngx_str_t               smtp_from;
     ngx_str_t               smtp_to;
 
+    ngx_str_t               cmd;
+
     ngx_uint_t              command;
     ngx_array_t             args;