* Correctly determine argv[0]

- If the function called is `posix_spawnp`, and the passed program name **does not** contain a slash, the path is used unmodified, and the path to the directory containing the program on $PATH is prepended to the program's path and assigned to `argv[0]. If the program name **does** contain a slash, the path is absolutised relative to CWD, and assigned to argv[0]

    - If the function called is `posix_spawn`, the behaviour is as described above in the case where path contains a slash

* Add initial tests

src/platform/linux/mod.rs

@@ -1,6 +1,8 @@
 #[cfg(target_arch = "x86_64")]
 use core::arch::asm;
 
+use alloc::string::String;
+
 use super::{Pal, types::*};
 use crate::{
     c_str::CStr,
@@ -800,7 +802,7 @@ impl Pal for Sys {
         fat: Option<&crate::header::spawn::posix_spawnattr_t>,
         argv: crate::iter::NulTerminated<*mut c_char>,
         envp: Option<crate::iter::NulTerminated<*mut c_char>>,
-        use_path: bool,
+        dir_ent_name: Option<String>,
     ) -> Result<pid_t> {
         todo!()
     }