src: Remove __init and __exit macro definitions.

src: Remove __init and __exit macro definitions.

Add nft_init and nft_exit functions, which calls _init and _exit
functions in main.c file. Remove __init and __exit macro definitions as
libnftables library will be created soon. Rename realm_table_init() and
realm_table_exit() functions to avoid ambiguity as
realm_table_rt_init(), realm_table_meta_init, realm_table_rt_exit() and
realm_table_meta_exit() in rt.c and meta.c files.

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>