builtins ignore signals with redirections

[zanchey]

As noted by @krobelus and me on Gitter, some builtins cannot be cancelled. This effectively livelocks the process running them, potentially infinitely.

Examples include:

• count </dev/urandom
• string trim </dev/stdin and math </dev/stdin, "because they use read_blocked which ignores EINTR from read(2). They ignore Control-C since fd6d814, which seems weird."

[ridiculousfish]

Yeah nice find!