[import] fix stdin/stdout pipe behavior in import/export tar/raw

The code existed in machinectl to use stdin/stdout if the path for
import/export tar/raw was empty or dash (-) but a check to
`fd_verify_regular` in importd prevented it from working.

Update the check instead to explicitly check for regular file or
pipe/fifo.

Fixes #14346

(cherry picked from commit 1209ef94)
(cherry picked from commit cfced59a)