Bugfix and FRSH thread function generalization.

Bugfix and FRSH thread function generalization.

Some bugs introduced in my previous commit have been fixed. More
important, some functions related to thread creation and management in
FRSH have been moved from resource managers to frsh_forb.

These functions are, for example frsh_thread_create_in_background, or
frsh_{set, get}_service_thread_data. This avoid code duplication inside
resource managers.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>