revamp acpitable parsing and allow to specify complete (headerful) table

This patch almost rewrites acpi_table_add() function
(but still leaves it using old get_param_value() interface).
The result is that it's now possible to specify whole table
(together with a header) in an external file, instead of just
data portion, with a new file= parameter, but at the same time
it's still possible to specify header fields as before.

Now with the checkpatch.pl formatting fixes, thanks to
Stefan Hajnoczi for suggestions, with changes from
Isaku Yamahata, and with my further refinements.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Cc: John Baboval <john.baboval@virtualcomputer.com>
Cc: Blue Swirl <blauwirbel@gmail.com>
[yamahata@valinux.co.jp: fix compile error, comment fallthrough]
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
