libnftables: Fix for input without trailing newline

libnftables: Fix for input without trailing newline

Input parser implementation requires a newline at end of input,
otherwise the last pattern may not be recognized correctly.

If input comes from a file, the culprit was YY_INPUT macro not expecting
the last line not ending with a newline, so the last word wasn't
accepted. This is easily fixed by checking for feof(yyin) in there. A
simple test case for that is:

| echo -en "table ip t {\nchain c {\n}\n}" >/tmp/foo
| nft -f /tmp/foo

Input from a string buffer is a bit more tricky: The culprit here is
that detection of classid pattern is done by checking the character
following it which makes it impossible to sit right at end of input and
I haven't found an alternative to that. After dropping the manual
newline appending when combining argv into a single buffer in main(),
a rule like this won't be recognized anymore:

| nft add rule ip t c meta priority feed:babe

Since a direct call to run_cmd_from_buffer() via libnftables bypasses
the sanitizing done in main() entirely, it has to happen in libnftables
instead which means creating a newline-terminated duplicate of the input
buffer.

Note that main() created a buffer one byte longer than needed since it
accounts for whitespace at end of each argv but doesn't add it to the
buffer for the last one, so buffer length is reduced by two bytes
instead of just one although only one less character is printed into it.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>