make usage of concrete file paths more robust

examples/isqrt/why3session.xml

share/why3session.dtd

src/driver/call_provers.ml

src/driver/driver_lexer.mll

src/driver/whyconf.ml

src/session/controller_itp.ml

src/session/itp_server.ml

src/session/session_itp.ml

src/session/session_itp.mli

src/tools/why3replay.ml

src/util/sysutil.ml

src/util/sysutil.mli

src/why3session/why3session_html.ml

src/why3session/why3session_info.ml

src/why3session/why3session_latex.ml

src/why3session/why3session_update.ml