Fix header FSM in webserver, fix Windows uploads

[earlephilhower]

When a file upload ends in \r\n (i.e. a Windows formatted text file) the
sequence of bytes on the wire is \r\n\r\n----...boundary-marker....

When the FSM in Webserver was evaluating the stream, that 2nd \r would
be written as valid data and the FSM parser would be reset and see \n
as the 1st character, which wouldn't match. This would a) add an extra
\r to uploaded files, and b) cause uploads to hang.

Fix by checking on a header FSM mismatch if the next character input
could possibly match our marker, and if so handle it properly.

Fixes #7803

[earlephilhower]

@luc-github can you please try this and report back?

[luc-github]

I have just tested and issue is solved - thank you

C:\Users\luc\Desktop\webserver>curl -v -L -F "[email protected]" http://192.168.1.75/upload
*   Trying 192.168.1.75...
* TCP_NODELAY set
* Connected to 192.168.1.75 (192.168.1.75) port 80 (#0)
> POST /upload HTTP/1.1
> Host: 192.168.1.75
> User-Agent: curl/7.55.1
> Accept: */*
> Content-Length: 36006
> Expect: 100-continue
> Content-Type: multipart/form-data; boundary=------------------------4738aa4a9bc8107f
>
* Done waiting for 100-continue
< HTTP/1.1 200 OK
< Content-Type: text/plain
< Content-Length: 9
< Connection: keep-alive
< Keep-Alive: timeout=2000
<
ok boomer* Connection #0 to host 192.168.1.75 left intact

Connecting to WIFI_OFFICE_B2G
.......
Connected! IP address: 192.168.1.75
HTTP server started
Starting upload
Filename gpl.txt
..................
Ending upload