command: skip nonregular files

[ahmethakanbesel]

Resolves #498

Changes are made:

• cp command checks file mode bits before trying to copy. If the file is a special file, cp do not copy that file. It neither returns an error nor prints a message. The copy process continues as usual.
• As sync uses cp  in the background, it behaves like cp, so it does not sync special files.

[ahmethakanbesel]

I do not remember if we ever discussed this but what about outputting some warnings to users?

For example; a cp to a single fifo file exits with 0 without any output which kinda indicates like if it would successfully copied;

~/src/github.com/peak/s5cmd fix-special-files*
; file fifoo
fifoo: fifo (named pipe)

~/src/github.com/peak/s5cmd fix-special-files*
; s5cmd cp fifoo s3://some-bucket/selman/test/; echo $?
0

~/src/github.com/peak/s5cmd fix-special-files*
; s5cmd ls 's3://some-bucket/selman/test/*'
ERROR "ls s3://some-bucket/selman/test/*": no object found

What do you think @ilkinulas?

I added error messages for skipped objects. It can be good to show information messages. I can revert if it is unnecessary. https://github.com/peak/s5cmd/pull/618/commits/da3ccd44ed37ae66b0ac5465acc6ff8ae14c1ad1