include/sem: Allow semaphores to be created in arbitrary locations

include/sem: Allow semaphores to be created in arbitrary locations

The current implementation cannot be used to created semaphores outside
of $TOOLBOX_HOME/sem. This makes it impossible to share semaphores with
scripts that are executed as a different user.
This commit changes the semaphore implementation so that only semaphore
names that don't contain slashes will be created in $TOOLBOX_HOME/sem.