Add support for the same output format as the bash script,
and use its implementation instead of the previous one.

I opted to do such patch in order to have a single script
responsible for parsing Documentation/features and
produce different outputs.

As someone may rely on the past format, which is easy
to parse it, get_feat.pl now gains a new command with
the same output format as the previous script.

As a side effect, the perl script is a lot faster, as it reads
each file only once, instead of parsing files several times
via a for command and grep commands inside it.

This patch also changes the features list order to be
case-insensitive, in order to better match the output of
the existing script.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a97f49677805ad4e6b982d02c0db8c9dfbbd20a6.1606748711.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>