X wcfs: client: os: Factor syserr -> string into _sysErrString

Currently the code to convert `int err` or errno into string is usde
only in _pathError, but in the next patches we'll need it to also handle
error from pthread_atfork. -> Factor-out to separate function.

wcfs/client/wcfs_misc.cpp

@@ -45,6 +45,7 @@ global<error> ErrClosed = errors::New("file already closed");
 
 // TODO -> os.PathError + err=syscall.Errno
 static error _pathError(const char *op, const string &path, int syserr);
+static string _sysErrString(int syserr);
 
 int     _File::fd() const   { return _fd; }
 string  _File::name() const { return _path; }
@@ -128,10 +129,16 @@ error _File::_errno(const char *op) {
 // _pathError returns os.PathError-like for op/path and system error
 // indicated by syserr.
 static error _pathError(const char *op, const string &path, int syserr) {
+    // TODO v(_sysErrString(syserr)) -> v(syscall.Errno(syserr))
+    return fmt::errorf("%s %s: %s", op, v(path), v(_sysErrString(syserr)));
+}
+
+
+// _sysErrString returns string corresponding to system error syserr.
+static string _sysErrString(int syserr) {
     char ebuf[128];
     char *estr = strerror_r(syserr, ebuf, sizeof(ebuf));
-
-    return fmt::errorf("%s %s: %s", op, v(path), estr); // TODO estr -> syscall.Errno
+    return string(estr);
 }
 
 }   // os::