pthread_mutex_trylock return value on recursion

[Konstantin Belousov]

Hi.
In the lib/tdb/common/mutex.c, the tdb_runtime_check_for_robust_mutexes()
function, (AFAIU) which tests the implementation of shared robust
mutexes for suitability, has the following fragment:
https://github.com/samba-team/samba/blob/master/lib/tdb/common/mutex.c#L928

	ret = pthread_mutex_trylock(m);
	if (ret != EDEADLK) {

The preconditions are that the mutex was locked with EOWNERDEAD result,
so the current thread already owns the m' lock, and the call I cited
attempts to recursively lock the owned mutex.

According to POSIX, pthread_mutex_trylock() must always return EBUSY in
case the lock cannot be granted.  EDEADLK is only allowed for blocking
pthread_mutex_lock().  Glibc implementation indeed returns
EDEADLK for errorcheck mutexes, but this seems to be non-conformant.

Would you mind to check for both EDEADLK and EBUSY there, or add a
configure test and select between EBUSY and EDEADLK at compile time ?
According to the comment, the goal of the check is to distinguish
whether libpthread is loaded, so either non-zero error checked is good.

Thanks.
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