[REFACTOR] Exit for SIGPIPE signal without leaks

[itislu]

By setting up a signal handler that cleans and exits for SIGPIPE, we exit with the first write() to a broken pipe without trying over and over again.
To avoid memory leaks, allocations in builtins that have not been freed before a call to write() have to be safed into the shell struct so that they can be freed in the signal handler.

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Also:

• Use ft_bzero() to init shell struct to 0.
• Shorten abort message.
• Make environment list operation functions more memory save.
• Fix sa.sa_flags not matching with the function pointers.

[itislu]

Aahh it's fruststrating that the tester behaves differently in the GitHub Action environment...
I just cannot figure out why our minishell doesn't print the SIGPIPE error messages from external commands even though the signal handler for SIGPIPE gets reset to SIG_DFL.
I also cannot reproduce when bash prints them in the first place, because locally it never does even though the same version of bash is installed.

[itislu]

@LeaYeh Please check if the updates to the README about SIGPIPE are okay for you