mailbox/:  Bugfixes and cleanup

- Fix bugs in mailboxAlloc(), mailboxCount(), mailboxFree(),
  mailboxReceive(), mailboxSend(): All these functions were subject to
  race conditions with concurrent calls to mailboxAlloc() and/or
  mailboxFree().

- Fix bug in mailboxAlloc():  If the 'sender' or 'receiver' semaphores
  could not be created, the function returned without releasing
  'mboxtabsem'.

- Fix bug in mailboxInit():  Function did not return SYSERR if the
  mboxtabsem could not be created.

- Improve function comments

- Remove unnecessary checks of semfree() and memfree() return values in
  mailboxFree()