Do not use -I preprosessor directive to locate local headers

Using relative paths makes it easier to tell find local headers and
tell them from external headers.

Also remove unmaintained, possibly not up to date hints about why
headers are included.
Add empty lines to improve readability

Tightly interleaved doxygen data and code made it unnecessarily
hard to browse the code.
Move datapipes variables from mce.h to datapipe.c

Duplicates were generated in to every object file that was compiled
from source code that included mce.h. I guess this a) compiled
because the structures are not initialized b) worked because the
runtime linker resolved refs from the plugins to use variables
compiled into mce main binary.

Leave extern declarations to the header, but define the actual
variables only once in the datapipe module.

[mce] Move datapipes variables from mce.h to datapipe.c. Fixes JB#18917
Move datapipe initialization and cleanup to datapipe.c

Makes the still huge mce main() function a bit more readable.
Remove redundant #include directives

Also remove unmaintained, possibly not up to date hints about why
headers are included.
Merge pull request #169 from spiiroin/data_in_headers

Include file fixes