bird1: rename get_bool() to get_a_bool() in bird{4,6}-lib.sh

bird1: rename get_bool() to get_a_bool() in bird{4,6}-lib.sh

This patch renames the get_bool() function, implemented in the
bird4-lib.sh and bird6-lib.sh files (respectively, for the IPv4 and
the IPv6 versions of the package), as well as all the calls to it.
This way, we avoid a function name collision with the one provided by
file /lib/functions.sh, which is slightly different and caused an
out-of-memory error when parsing the bird4/bird6 UCI config files.

Fixes #920.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>