MFC r281436 (by mjg@):

fd: remove filedesc argument from fdclose

Just accept a thread instead. This makes it consistent with fdalloc.

No functional changes.

MFC r285483: pipe_direct_write: Fix mismatched pipelock/unlock

If a signal is caught in pipelock, causing it to fail, pipe_direct_write
should not try to pipeunlock.

Approved by: markj (mentor)
Sponsored by: EMC / Isilon Storage Division

MFC r278145:
Fix use after free in pipe_dtor().

MFC r275260:
Remove lock recursion for the pipe pair mutex, and disable the
recursion on mutex initialization.

MFC r274023:
When other end of the pipe closed during the write, but some bytes
were written, return short write instead of EPIPE.

MFC r265206:

Ignore the error from pipespace_new when creating a pipe.

It can fail if pipe map is exhausted (as a result of too many pipes created),
but it is not fatal and could be provoked by unprivileged users. The only
consequence is worse performance with given pipe.

MFC r285483: pipe_direct_write: Fix mismatched pipelock/unlock

If a signal is caught in pipelock, causing it to fail, pipe_direct_write
should not try to pipeunlock.

Approved by: markj (mentor)
Sponsored by: EMC / Isilon Storage Division

MFC r278145:
Fix use after free in pipe_dtor().

MFC r275260:
Remove lock recursion for the pipe pair mutex, and disable the
recursion on mutex initialization.

MFC r274023:
When other end of the pipe closed during the write, but some bytes
were written, return short write instead of EPIPE.

MFC r265206:

Ignore the error from pipespace_new when creating a pipe.

It can fail if pipe map is exhausted (as a result of too many pipes created),
but it is not fatal and could be provoked by unprivileged users. The only
consequence is worse performance with given pipe.