SourceLayout: src/base, take 0 with tweaks

Added src/base directory for fundamental, commonly-used code pieces that are
not large enough to warrant their own directories. Currently base/ contains
the beginning of AsyncCalls hierarchy, which may eventually get its own
directory.

Removed src/ip from the top-level SUBDIRS list. Made cf_gen compile without
libip.

Started building makefile includes, to be used in subdir makefiles:
src/Common.am and src/TestHeaders.am.