[BUGFIX]: Amended free_command() to return if the command_t pointer argument is null
[CLEANUP]: Changed set_sys_env_var() to return void instead of int since we can't really do anything about an error anyways, we just report it and move on
[CLEANUP]: Added comments to execute_builtins() for clarity. Amended edit-env and add-env usage messages.
[FEATURE]: Added import_sys_env_var() to env.c to import system environment variables to builtin-shell environment variables. Changed set_sys_env_var()'s name to import_sys_env_var() to be consistent with the new naming scheme. Amended env.h to reflect these changes. Use import_sys_env_var() to get the actual home and path of the user instead of the defaults.
[BUGFIX]: The shell should ignore SIGTERM, spawned processes should go with the default handler. Only strip newlines if there's actually a string. Ignore input that's only a newline.
[CLEANUP]: Remove useless 0 length input handling code
[CLEANUP]: Move strip_newline() into parse instead of the main loop.
[CLEANUP/NEWFILES]: Added helpers.c/helpers.h. Moved shell_error(), copy_string(), strip_newline(), and count_token() from shell.c and to_lower_varname() and to_upper_varname() from env.c to helpers.c and helpers.h
[CLEANUP]: Renamed do_replace() to do_var_name_substitution()