support pipefail

[gdetal]

This commit adds the option CMD_LIB_PIPE_FAIL. If set to 1 this will cause both run_cmd!() and run_fun!() to return an error when at least one command fails. This actually implements a similar behavior as -o pipefail from bash.

Example:

• without pipefail (default behavior):

run_cmd!(cat` /non-existant | wc -l)?; // returns Ok(())

• with pipefail:

proc_env_set!(CMD_LIB_PIPE_FAIL = 1);
run_cmd!(cat /non-existant | wc -l)?; // Returns Err(_)