cri-getconfig: Break out exec_resetting_sigint.

cri-getconfig: Break out exec_resetting_sigint.

Move this oddity (and the associated comment) from
standalone-generate-dump-flight-runvars to cri-getconfig.  We are
going to want it elsewhere.

We put this in cri-getconfig because that is the one library of
generic shell functions which everything includes.  Perhaps this file
is misnamed.

No overall functional change.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>