Add --fileprefix option

Suppose there's a library whose clients are supposed to include
<somelib/api.h>, and api.h contains not much meat, but just includes
myriad other somelib/*.h.

Suppose you have somelib's sources unpacked. If you use c2hsc to process
include/somelib/api.h, there won't be much output. But if you specify
--fileprefix='./include/somelib', all the various headers included from
include/somelib will be processed, and there will be much more useful
output.