app-tools: Use s///g in seddery

Substitute even multiple occurrences in a single line.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
app-tools: -U various unwanted predefines

Find the list of things to undefine by running rumpmake on
rumptools/mk.conf: we take everything that rumpmake's CPPFLAGS would
undefine and undefine that ourselves.

The result is that we never (for example) provide -D__linux__.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
app-tools: Pass -I.../rumpuser-xen/include

Applications may want the Xen or minios headers.  We pass that as the
last -isystem (because these headers aren't very namespace clean -
particularly, the minios headers are very bad).

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
callmain: Print something before and after main()

This makes it easier to find the application program's output.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>