cat builtin: permit interrupt during writing; more: avoid SIGPIPE

Change-Id: I883dbb359e2580779d7d9fc8e982fbe2a9180668
Merge "cat builtin: permit interrupt during writing; more: avoid SIGPIPE"
Upgrade to mksh 50.

Bug: 16569301
Change-Id: I0560d52066198e25d3f0e5f3e4b0b7a5cd4f60b4
Merge "Upgrade to mksh 50."