this allows to include the regexes in another file and apply
transformations and experiments.

using an empty group () is not defined in the posix spec, and as such
"undefined behaviour", even though it happened to work with both GLIBC
and MUSL libc, as well as with oniguruma's POSIX compatibility API.

we used this idiom as a trick when refactoring the regex parsing,
in order not to change the match indices of all the handler functions,
ignorant that this is not explicitly allowed by the spec.

to make future refactoring easier, we introduce a MGROUP1 macro that's
added to each match group index, so we have only a single knob to turn
in case a similar change becomes necessary again.

likewise

we already deal with leading whitespace before a command in a manual
way before comparing keywords.

rather than treating everything as case insensitive, we explicitly
allow upper/lowercase where it makes sense.

use one matching group rather than 3.

with the IPv4 regex simplification from 22f059d
we're back to max 15 match groups according to re2r analysis
(the most elaborate regex is the upstream one).

this is currently not included in the build system and needs to be
compiled by hand.

conf_regex.rl is generated from the output of conf_regex_print.c using
re2r (https://github.com/rofl0r/re2r).
if ragel is available on the build host, it is being used to generate
finite state machines from the regexes used by the config file parser
for an impressive speed boost, while only adding moderately to binary
size.
a stripped x86_64 tinyproxy binary compiled with -O2 is still only ~100KB.