Add warning message for locale/Unicode intermixing

This is explained in the added perldiag entry.
perlfunc: Fix typo
handy.h: Add missing parentheses to macro #define

These being missing caused 3d3a881c1b0eb9c855d257a2eea1f72666e30fbc
to have to be reverted.  It only shows up on platforms that don't have
an isblank() libc function.
Reinstate "Raise warnings for poorly supported locales"

This reverts commit 1244bd171b8d1fd4b6179e537f7b95c38bd8f099,
thus reinstating commit 3d3a881c1b0eb9c855d257a2eea1f72666e30fbc.
lib/locale.t: Display debug information under harness

This just changes any debugging (which is by default off) goes to
STDERR.