Add vmute member flag to mod_conference.

A 'vmute' flag can be applied to individual conferees when entering the
conference. When the flag is passed, conferees will enter the conference
with their video muted.

FS-7813 #resolve

src/mod/applications/mod_conference/mod_conference.c

@@ -12789,6 +12789,8 @@ static void set_mflags(const char *flags, member_flag_t *f)
 			if (!strcasecmp(argv[i], "mute")) {
 				f[MFLAG_CAN_SPEAK] = 0;
 				f[MFLAG_TALKING] = 0;
+			} else if (!strcasecmp(argv[i], "vmute")) {
+				f[MFLAG_CAN_BE_SEEN] = 0;
 			} else if (!strcasecmp(argv[i], "deaf")) {
 				f[MFLAG_CAN_HEAR] = 0;
 			} else if (!strcasecmp(argv[i], "mute-detect")) {