uftpd.py: Implement SITE for exec'd statements.

By @jdsmith

Various FTP servers implement SITE "to provide services to a client that
may not be universally supported by all FTP clients". This small
addition implements a SITE command which simply exec's Python
statements. Since FTP commands may not contain newlines, the commany
payload shall contain \x00 characters as line ends, whcih are
substituted by \n.

The only catch is that if the user supplies a SITE command which
blocks when exec'd, uftpd blocks as well.

README.md

@@ -13,6 +13,8 @@ bytecode, by placing it into the esp8266/modules folder and performing a rebuild
 or it must be compiled into bytecode using mpy-cross and loaded as an .mpy file.
 The frozen bytecode variant is preferred.
 
+## Limitations
+
 The server has some limitations:
 - Binary mode only
 - Limited multi-session support. The server accepts multiple sessions, but only
@@ -25,9 +27,13 @@ authentication may be added easily, if required.
 Even when the ftp server sits in background and can serve requests, **no
 foreground tasks should run at that time**, especially if they execute system calls.
 The effects is hardly to predict, although most of the time the device simply
-crashes.
+crashes. Also in turn, when using the SITE command, the code in the payload MUST NOT
+be blocking, because that will block the device.
 - ESP32 The server is supported from version='v1.9.3-575 on. That is the version
 which introduced webrepl.
+- The server uses a special setsocket option, which was introduced for webrepl
+and registers a callback handler. Both webrepl and uftpd use this mechanism and
+register their own exclusive handler. Therefore you canot use both at the same time.
 
 
 ## Start-up

uftpd.py

@@ -364,6 +364,12 @@ def exec_ftp_command(self, cl):
                     cl.sendall('250 OK\r\n')
                 except:
                     cl.sendall('550 Fail\r\n')
+            elif command == "SITE":
+                try:
+                    exec(payload.replace('\0','\n'))
+                    cl.sendall('250 OK\r\n')
+                except:
+                    cl.sendall('550 Fail\r\n')
             else:
                 cl.sendall("502 Unsupported command.\r\n")
                 # log_msg(2,