DESC
Initialize mutexes' attributes.
EDESC
SUSv3 says that default mutexes are of unspecified type. This can lead to
deadlocks turning into a return value EDEADLK, which we don't really want
since we don't check every single lock()/unlock() return value.

This patch initializes them properly by setting the mutexes to normal,
non-error-checking ones.
