Add an include-checking script

Add an include-checking script

This adds a new Python script that checks the header guards of all gdb
source files.  It enforces a fairly strict formatting and naming
scheme.

In particular, for a file "x/y-z.h" (relative to the repository root),
the include guard will be named "X_Y_Z_H".  Only the '#ifndef' form is
allowed, not "#if !defined(...)".  The trailing comment on the
"#endif" is also required.

The script also tries to update files that appear to have the required
lines if they are in the wrong form or use the wrong name.

Reviewed-By: Tom de Vries <tdevries@suse.de>