manpages: use an intercept to run mandb

If you build an image with lots of manpages in, then each package will
run mandb inside qemu-user at rootfs time.  This is a slow operation
and should be done once when all of the packages have installed using an
intercept instead.

The call to mandb has been changed too. mandb doesn't actually allow
the configuration file to be read from stdin so that was being ignored,
instead write the file to a temporary file and use that.

This means we then don't need to tell it where to search explicitly, and
it writes the indexes to the correct paths so we don't need to move
files afterwards either.

Sadly we do still need to run mandb inside qemu-user, as the underlying
database is a gdbm file and they are byte-order dependent.

For my test case of core-image-base with api-documentation
DISTRO_FEATURES and doc-pkgs IMAGE_FEATURES enabled, the performance
gain is significant:

  core-image-base do_rootfs  -1303.1s   -73.6%   1771.6s -> 468.5s

(From OE-Core rev: fbd8a57aa307bfda70a08cb78af3c97f05c39a3a)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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