cr: Ignore Ctrl-C while running shell programs

While running programs in an interactive subshell (cr shell), ignore
Ctrl-C in cr itself. This lets the child program handle Ctrl-C the best
way it sees fit without always terminating cr.

TEST=cr shell gdb # Press Ctrl-C => Neither gdb nor cr terminates.
NOTRY=true

Review URL: https://codereview.chromium.org/167373003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251662 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251662