Use tee(2) to avoid reading one byte at a time from stdin

[herbertx]

People have been complaining about the lack of the ability peek on pipes on Linux for years. However, there is actually a way to do that, through the system call tee(2). So dash should start using this to read stdin pipes in order to avoid the penalty of one read(2) per byte.

[herbertx]

https://lore.kernel.org/dash/cover.1714900988.git.herbert@gondor.apana.org.au/

[herbertx]

44b15ea