trafgen: parser: Check read access to file before invoking cpp

trafgen: parser: Check read access to file before invoking cpp

If a non-accessible (or non-existing) file is passed to trafgen and the
-p/--cpp option is used, the preprocessor will fail but trafgen will
continue running and producing follow-up errors messages:

	cpp: error: foo.conf: No such file or directory
	cpp: warning: ‘-x c’ after last input file has no effect
	cpp: fatal error: no input files
	compilation terminated.
	     0 packets to schedule
	Enabled kernel qdisc bypass
	     0 bytes in total
	Running! Hang up with ^C!

	Enabled kernel qdisc bypass
	TX,V2: 0.50 MiB, 256 Frames, each 2048 Byte allocated
	TX,V2: 0.50 MiB, 256 Frames, each 2048 Byte allocated

		   0 packets outgoing
		   0 bytes outgoing
		   0 sec, 0 usec on CPU0 (0 packets)
		   0 sec, 0 usec on CPU1 (0 packets)

To avoid this, check that the file is readable before passing it to the
preprocessor and error out if is not readable.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>