strerror thread safety

[Kevin Ryde]

        * error.c (scm_strerror): Use scm_i_misc_mutex around strerror since
        it's not thread safe.
        (scm_syserror): Use scm_strerror rather than SCM_I_STRERROR, to take
        advantage of this.
        * fports.c (scm_open_file): Use scm_strerror likewise.
        * filesys.c (scm_stat, scm_lstat): Ditto.

A program provoking the problem (in glibc at least),


(use-modules (ice-9 threads))

(define e1 9999)
(define s1 (strerror e1))

(define e2 7777)
(define s2 (strerror e2))

(begin-thread
 (while #t
   (display "-")
   (let ((s (strerror e1)))
     (if (not (string=? s1 s))
         (begin
           (format #t "oops, wrong s1, got ~s want ~s\n" s s1)
           (primitive-exit 1))))))

(while #t
  (display ".")
  (let ((s (strerror e2)))
    (if (not (string=? s2 s))
        (begin
          (format #t "oops, wrong s2, got ~s want ~s\n" s s2)
          (primitive-exit 1)))))

error.c.strerror-mutex.diff
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