srd: add support for SIGROKDECODE_PATH environment variable

srd: add support for SIGROKDECODE_PATH environment variable

The optional SIGROKDECODE_DIR environment variable accepts a single
directory specification. The SIGROKDECODE_PATH variable accepts a list
of directories to search protocol decoders. The list separator is
platform dependent (colon or semicolon). Empty items are explicitly
ignored. Both variables get evaluated. Behaviour remains backwards
compatible, just gets extended for improved usability.