maint: improve make src/fs-magic-compare

maint: improve make src/fs-magic-compare

* README-release: fix the `make` command, and mention how
to get the latest results without requring running a
system with the latest kernel.
* src/local.mk (src/fs-latest-magic.h): A new target to
document how/where to place the latest magic header.
(src/fs-kernel-magic): Adjust to include separately
downloaded header if available.
(src/fs-magic): Undefine MANPAGER as it may impact the
ability to pipe the output of man(1).
(fs-magic-compare): Don't echo the commands run as they're
distracting from the output which needs to be examined.