new components bsls_muteximp, bsls_muteximpl_pthread, bsls_muteximpl_win32
(heavily based on bcemt), and associated changes to bsls_platform

changes per review feedback:
  - bsls_muteximp is now bsls_bsllock
  - no separate "impl" components for win32 and pthread

rename 'BslLockProctor' to 'BslLockGuard'
[APPROVAL] review/bsls-mutex-drqs-41890133 to master