libnftables: Simplify nft_run_cmd_from_buffer footprint

libnftables: Simplify nft_run_cmd_from_buffer footprint

With libnftables documentation being upstream and one confirmed external
user (nftlb), time to break the API!

First of all, the command buffer passed to nft_run_cmd_from_buffer may
(and should) be const. One should consider it a bug if that function
ever changed it's content.

On the other hand, there is no point in passing the buffer's length as
separate argument: NULL bytes are not expected to occur in the input, so
it is safe to rely upon strlen(). Also, the actual parsers don't require
a buffer length passed to them, either. The only use-case for it is when
reallocating the buffer to append a final newline character, there
strlen() is perfectly sufficient.

Suggested-by: Harald Welte <laforge@gnumonks.org>
Cc: Laura Garcia Liebana <nevola@gmail.com>
Cc: Eric Leblond <eric@regit.org>
Cc: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>