tools: bpftool: support continuation lines in batch files

Add support for continuation lines, such as in the following example:

    prog show
    prog dump xlated \
        id 1337 opcodes

This patch is based after the code for support for continuation lines
from file lib/utils.c from package iproute2.

"Lines" in error messages are renamed as "commands", as we count the
number of commands (but we ignore empty lines, comments, and do not add
continuation lines to the count).

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>