Changing standard input detection from fstat to istty

## `-t` not supported in MSYS

### Problem

`-t` isn't supported in MSYS

This is a problem because

* it's useful to pipe input to `ack` with `echo test | ack test`
* MSYS is sometimes used instead of Cygwin as a pre-configured `make` environment, f.e. by devkitPro
* `ack` is useful for make because it can highlight errors with `make -r | ack --passthru error`



## `-t` not detecting `S_ISREG`

`-t` doesn't detect the `fstat` mode `S_ISREG`


This is a problem because

* it's useful to redirect input to `ack` with `ack test < input.txt`



## Test

The output from

	echo test > input.txt

	perl -e 'print "" . (-p STDIN) ? 1 : 0 . "\n"'
	echo test | perl -e 'print "" . (-p STDIN) ? 1 : 0 . "\n"'
	cat input.txt | perl -e 'print "" . (-p STDIN) ? 1 : 0 . "\n"'
	perl -e 'print "" . (-p STDIN) ? 1 : 0 . "\n"' < input.txt

	perl -e 'print "" . (not -t STDIN) ? 1 : 0 . "\n"'
	echo test | perl -e 'print "" . (not -t STDIN) ? 1 : 0 . "\n"'
	cat input.txt | perl -e 'print "" . (not -t STDIN) ? 1 : 0 . "\n"'
	perl -e 'print "" . (not -t STDIN) ? 1 : 0 . "\n"' < input.txt

is
	
	uname -a
	Linux ubuntu 3.8.0-23-generic #34-Ubuntu SMP Wed May 29 20:22:58 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
	perl --version
	This is perl 5, version 14, subversion 2 (v5.14.2) built for x86_64-linux-gnu-thread-multi
	0
	1
	1
	0


	0
	1
	1
	1

	uname -a
	CYGWIN_NT-6.1-WOW64 pc 1.7.18(0.263/5/3) 2013-04-19 10:39 i686 Cygwin
	perl --version
	This is perl 5, version 14, subversion 2 (v5.14.2) built for cygwin-thread-multi-64int
	0
	1
	1
	0

	0
	1
	1
	1

	uname -a
	MSYS_NT-6.1 PC 1.0.18(0.48/3/2) 2012-11-21 22:34 i686 Msys
	perl --version
	This is perl, v5.8.8 built for msys-64int
	0
	0
	0
	0

	0
	1
	1
	1