Empty path shall result with /, not with an empty string. Even if it is
handled by some backends, it's a kind of a lie, especially, when additional
path from the request come in.

XXX - is that really the place to fix it?

XXX:
 * improve it more
 * adapt tests

Test backend needs to mimic all possible real backend cases.

Following advice from https://luajit.org/download.html use recent commit id,
but rely on github archive.

Drop obsoleted patch, enforce --with-shared-termcap-library

Also use directly archive with patches.

Changes:
 * use xz archive
 * stabilize patch name
 * update patch to current version

Expose pkgconfig configuration so that other parts can easily fetch it.

See https://bugs-devel.debian.org/cgi-bin/bugreport.cgi?bug=1077190

Drop fmhttp, as it's not needed to make http calls from our deployments, and
the updated version requires much more dependencies for this feature.