core: Add ExecSearchPath parameter to specify the directory relative to which binarie...

core: Add ExecSearchPath parameter to specify the directory relative to which binaries executed by Exec*= should be found

Currently there does not exist a way to specify a path relative to which
all binaries executed by Exec should be found. The only way is to
specify the absolute path.

This change implements the functionality to specify a path relative to which
binaries executed by Exec*= can be found.

Closes #6308