Add support for adjusting -symbollength

[jjlawren]

The default -symbollength used by rtlamr is 72, which defines a sample rate that may be higher than necessary for some users. A lower sample rate can reduce CPU consumption without impacting functionality.

Please see discussion here and documentation here.

In my case I can use a sample length of 8 for an r900 water meter with a significantly reduced CPU load.

[jjlawren]

Just noticed the ENVAR name used by rtlamr is slightly different and this can be simplified a bit. Will adjust.

[jjlawren]

Updated the script to let rtlamr rely on the environment variables it already supports. No need to explicitly set the flags where the default isn't overridden.