sh: Remove racy test case for read builtin.

This test case sometimes fails because of an EINTR-related race condition.
Fixing this race condition likely requires an extra system call per byte,
which would make the read builtin even slower than it already is, or very
complicated trickery. Therefore, remove the test case for now.
Similar to 233760 and 236717, export some more useful info about the
kernel-based POSIX semaphore descriptors to userland via procstat(1) and
fstat(1):
- Change sem file descriptors to track the pathname they are associated
  with and add a ksem_info() method to copy the path out to a
  caller-supplied buffer.
- Use the fo_stat() method of shared memory objects and ksem_info() to
  export the path, mode, and value of a semaphore via struct kinfo_file.
- Add a struct semstat to the libprocstat(3) interface along with a
  procstat_get_sem_info() to export the mode and value of a semaphore.
- Teach fstat about semaphores and to display their path, mode, and value.

MFC after:	2 weeks