include/inst: Make subsequent calls to inst_running() return false

include/inst: Make subsequent calls to inst_running() return false

When an instance is instructed to stop, a semaphore is increased to
signal this condition. The inst_running() function uses sem_trywait()
on this semaphore, returning false if the semaphore could be passed,
thus telling the caller that the instance should be stopped.
However, since sem_trywait() decreases the semaphore, subsequent calls
to inst_running() will return true again.
This commit changes inst_running() to increase the semaphore in case
it was decreased by sem_trywait(). However, there remains a small
window of opportunity where the function may return different values
to different callers if multiple processes from the same instance call
the function at the same time.