… other related improvements

- added compress-level parameter to select a compression level for
  file writing and changed the default for lz4 compression to do a
  better job compressing.  finished splitting glz input routines
  apart from glz output routines and did a bit of other restructuring.
  removed gzxfile struct-as-bytevector wrapper and moved its fd
  into glzFile.  moved DEACTIVATE to before glzdopen_input calls
  in S_new_open_input_fd and S_compress_input_fd, since glzdopen_input
  reads from the file and could block.  the compress format and now
  level are now recorded directly the thread context.  replaced
  as-gz? flag bit in compressed bytevector header word with a small
  number of bits recording the compression format at the bottom of
  the header word.  flushed a couple of bytevector compression mats
  that depended on the old representation.  (these last few changes
  should make adding new compression formats easier.)  added
  s-directory build options to choose whether to compress and, if
  so, the format and level.
    compress-io.h, compress-io.c, new-io.c, equates.h, system.h,
    scheme.c, gc.c,
    io.ss, cmacros.ss, back.ss, bytevector.ss, primdata.ss, s/Mf-base,
    io.ms, mat.ss, bytevector.ms, root-experr*,
    release_notes.stex, io.stex, system.stex, objects.stex
- improved the effectiveness of LZ4 boot-file compression to within
  15% of gzip by increasing the lz4 output-port in_buffer size to
  1<<18.  With the previous size (1<<14) LZ4-compressed boot files
  were about 50% larger.  set the lz4 input-port in_buffer and
  out_buffer sizes to 1<<12 and 1<<14.  there's no clear win at
  present for larger input-port buffer sizes.
    compress-io.c
- To reduce the memory hit for the increased output-port in_buffer
  size and the corresponding increase in computed out_buffer size,
  one output-side out_buffer is now allocated (lazily) per thread
  and stored in the thread context.  The other buffers are now
  directly a part of the lz4File_out and lz4File_in structures
  rather than allocated separately.
    compress-io.c, scheme.c, gc.c,
    cmacros.ss
- split out the buffer emit code from glzwrite_lz4 into a
  separate glzemit_lz4 helper that is now also used by gzclose
  so we can avoid dealing with a NULL buffer in glzwrite_lz4.
  glzwrite_lz4 also uses it to writing large buffers directly and
  avoid the memcpy.
    compress-io.c
- replaced lz4File_out and lz4File_in mode enumeration with the
  compress format and inputp boolean.  using switch to check and
  raising exceptions for unexpected values to further simplify
  adding new compression formats in the future.
    compress-io.c
- replaced the never-defined struct lz4File pointer in glzFile
  union with the more specific struct lz4File_in_r and Lz4File_out_r
  pointers.
    compress-io.h, compress-io.c
- added free of lz4 structures to gzclose.  also changed file-close
  logic generally so that (1) port is marked closed before anything is
  freed to avoid dangling pointers in the case of an interrupt or
  error, and (2) structures are freed even in the case of a write
  or close error, before the error is reported.  also now mallocing
  glz and lz4 structures after possibility of errors have passed where
  possible and freeing them when not.
    compress-io.c,
    io.ss
- added return-value checks to malloc calls and to a couple of other
  C-library calls.
    compress-io.c
- corrected EINTR checks to look at errno rather than return codes.
    compress-io.c
- added S_ prefixes to the glz* exports
    externs.h, compress-io.c, new-io.c, scheme.c, fasl.c
- added entries for mutex-name and mutex-thread
    threads.stex