the dynamic input buffer resize code (fillbuf()) is broken as
the calling code assumes that memory wont relocate. instead
of trying to work out all the cases where this happens, i'm
getting rid of fillbuf() and just read the whole file into
memory in setsource().
the bug could be reproduced with something as simple as:
@{for(i in `{seq 1 10000}){echo $i ', \'; }} | cpp
foo.c includes bar/bar.h, which includes "baz.h"; it wants bar/baz.h
meanwhile, it also includes meh/quux.h, which includes "baz.h"; it wants meh/baz.h
