Add feature to specify custom environment variables for plugins.
* plugin-runner.c (plugin): New members "environ" and "envc" to contain possible custom environment. (getplugin): Return NULL on failure instead of doing exit(); all callers changed. (add_to_char_array): New helper function for "add_argument" and "add_environment". (addargument): Renamed to "add_argument". Return bool. Call "add_to_char_array" to actually do things. (add_environment): New; analogous to "add_argument". (addcustomargument): Renamed to "add_to_argv" to avoid confusion with "add_argument". (main): New options "--global-envs" and "--envs-for" to specify custom environment for plugins. Print environment for plugins in debug mode. Use asprintf instead of strcpy and strcat. Use execve() for plugins with custom environments. Free environment for plugin when freeing plugin list.