library: ensure 'XTRA_PROCPS_DEBUG' works in real life

library: ensure 'XTRA_PROCPS_DEBUG' works in real life

This commit attempts to smooth some wrinkles impacting
any future libprocps user exploitation. The 2 problems
relate exclusively to our XTRA_PROCPS_DEBUG provision.

1. The 'xtra-procps-debug.h' header had an include for
'procps-private.h', which was not an installed header.
So the STRINGIFY macros will now be embedded directly.

2. Each of the new api headers referenced '<proc/...>'
rather than '<procps/...>' for the debugging #include.
So, we must drop that prefix in favor of a quoted file
name so that debugging builds work regardless of where
that 'xtra-procps-debug.h' header happens to be found.

Reference(s):
. Jun, 2020 - changed target install directory
commit d64755ea5f48295f5b064da0ea667ee3503b4cd8
. Sep, 2018 - #include migrated from procps.h
commit fab37662efe92650063a45e6b20eda09afe07e14

Signed-off-by: Jim Warner <james.warner@comcast.net>