[scripts] lint-whitespace: use perl instead of grep -P #12097

MacOS does not support grep -P out of the box. This change makes it easier for developers to check for whitespace problems locally.

Based on this and this Stack Exchange answer.

Tested with:

export TRAVIS_COMMIT_RANGE='fe78c9a...62e0453'
contrib/devtools/lint-whitespace.sh 
This diff appears to have added new lines with tab characters instead of spaces.
The following changes were suspected:

diff --git a/src/test/bignum_tests.cpp b/src/test/bignum_tests.cpp
@@ -0,0 +1,110 @@
+	num.setint64(n);

Tested (macOS) ACK 40b17f5f9106e572420dada04c15ec452a8f4f41

Concept ACK

On NetBSD 7.0, "grep -P" is also not supported, so this change can be helpful there too (and maybe other platforms as well?).

Concept ACK

Please, next time someone writes a script like this, use Python. Shell script is terrible for making portable tools. Every time someone introduces a shell script we go through countless iterations to make it compatible with every shell and OS tool environment since the 80's.

(nevertheless, concept ACK)