Fix size_t/off_t mixup in std::filesystem.

[danalbert]

Summary

ftruncate takes an off_t, not a size_t.

[mclow.lists]

reference: http://pubs.opengroup.org/onlinepubs/009604499/functions/ftruncate.html

[danalbert]

Added reference to spec in comment.

[mclow.lists]

this looks fine to me; but I'd like @EricWF to chime in here.

src/filesystem/operations.cpp

file_status posix_lstat(path const& p, StatT& path_stat, error_code* ec) {
   return create_file_status(m_ec, p, path_stat, ec);
 }
 
 file_status posix_lstat(path const& p, error_code* ec) {
   StatT path_stat;
   return posix_lstat(p, path_stat, ec);
 }
 
-bool posix_ftruncate(const FileDescriptor& fd, size_t to_size, error_code& ec) {
+// http://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html
+bool posix_ftruncate(const FileDescriptor& fd, off_t to_size, error_code& ec) {
   if (::ftruncate(fd.fd, to_size) == -1) {
     ec = capture_errno();
     return true;
   }
   ec.clear();
   return false;
 }