--attach=yes fails with -z0

[rumba]

The --attach=yes option will fail to start the client if the --compress=0 option is used.

For example xpra start --attach=yes --start=xterm --daemon=no -z0 will produce:

xpra initialization error:
 no such option: --compression_level

[Antoine Martin]

Preface: unless your setup is very unusual (ie: a very low power CPU, or your installation is missing the lz4 compressor), then setting compression to zero will do absolutely nothing.

Fixes:

• r20050: support more command line option aliases for compression level
• r20051: rewrite command line arguments when generating command lines for servers potentially older then the current version (ie: via ssh)

r20051 also includes a fix for an unintended variable name shadowing bug

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1930