Manual pages: Standardize on EXIT STATUS as section title

Manual pages: Standardize on EXIT STATUS as section title

There is quite some value (in terms of readability and user
expectations) if consistent names are used for the sections
within manual pages. This patch is one of a series to bring
about this consistency.

Currently we have EXIT STATUS (18), EXIT CODES (3), RETURN CODE (7),
RETURN CODES (1), or RETURN VALUE (4 instances in pages that document
commands, rather than functions).

Let's standardize on the EXIT STATUS (which is also what is
suggested in man-pages(7), and is the POSIX terminology).

A subsequent patch will clean up corresponding miswordings in
manual page text.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>