Documentation/adding-syscalls.txt: convert it to ReST markup

Convert adding-syscalls.txt to ReST markup and add it to the
development-process book:

- add extra lines to make Sphinx to correctly parse paragraphs;
- use quote blocks for examples;
- use monotonic font for dirs, function calls, etc;
- mark manpage pages using the right markup;
- add cross-reference to SubmittingPatches.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>