specify connection attributes using the connection string

[totaam]

Similar to #1160 / #2424, should re-use the same syntax, ie:

xpra attach \
  tcp://127.0.0.1:10000/,cork=0,nodelay=1,encryption=AES:keyfile=key.txt,auth=file:filename=password.txt

[totaam]

We already have #2460, but the syntax above is better for shell compatibility as it does not require quotes. (no &)

Also, support for encryption options needs adding.

[totaam]

Done in r26644 (+r26645 typo), with fixes and improvements in r26647, r26648, r26650.

encryption=AES is assumed if the display string contains keyfile or keydata.
keydata can be used to specify the encryption key data inline, ie:

xpra attach tcp://user:[email protected]:10000/keydata=0123456789ABCDEF

The keydata can also be specified as hex:

xpra attach tcp://antoine:[email protected]:10000/keydata=0x30313233343536373839414243444546

Or using a keyfile:

echo -n "0123456789ABCDEF" > ./aes.txt
xpra attach tcp://antoine:[email protected]:10000/keyfile=./aes.txt