feat: Exec

[Oktosha]

Better exec:

proper return codes

• For syntax errors
• For libraries failing kotishell: failed malloc, failing pipes, ...
• For built-ins
• For child processes (normal and built-ins):
  • check WIFEXITED
  • also check errno for execve to get precise error cause
  • also handle death by signal

[Oktosha]

Valgrind behaves weirdly probably due to unclosed pipes. Every forked child receives all the pipes of the parent via fork but interacts only with the 2 file descriptors passed to l_ex_fork. A good child must close all the pipes it doesn't use.

[Oktosha]

Add dup2 error handling