Adopt Rust libc _CS_PATH

This is now supported directly by the libc crate - no need for fish
to expose this via C.

src/env/environment.rs

@@ -569,13 +569,12 @@ fn setup_user(vars: &EnvStack) {
 }
 
 pub(crate) static FALLBACK_PATH: Lazy<&[WString]> = Lazy::new(|| {
-    use crate::libc::_CS_PATH;
     // _CS_PATH: colon-separated paths to find POSIX utilities
-    let buf_size = unsafe { confstr(_CS_PATH(), std::ptr::null_mut(), 0) };
+    let buf_size = unsafe { confstr(libc::_CS_PATH, std::ptr::null_mut(), 0) };
     Box::leak(
         (if buf_size > 0 {
             let mut buf = vec![b'\0' as libc::c_char; buf_size];
-            unsafe { confstr(_CS_PATH(), buf.as_mut_ptr(), buf_size) };
+            unsafe { confstr(libc::_CS_PATH, buf.as_mut_ptr(), buf_size) };
             let buf = buf;
             // safety: buf should contain a null-byte, and is not mutable unless we move ownership
             let cstr = unsafe { CStr::from_ptr(buf.as_ptr()) };

src/libc.c

@@ -7,7 +7,7 @@
 #include <sys/mount.h>  // MNT_LOCAL
 #include <sys/resource.h>
 #include <sys/statvfs.h>  // ST_LOCAL
-#include <unistd.h>       // _CS_PATH, _PC_CASE_SENSITIVE
+#include <unistd.h>       // _PC_CASE_SENSITIVE
 
 uint64_t C_ST_LOCAL() {
 #if defined(ST_LOCAL)
@@ -17,14 +17,6 @@ uint64_t C_ST_LOCAL() {
 #endif
 }
 
-int C_CS_PATH() {
-#if defined(_CS_PATH)
-    return _CS_PATH;
-#else
-    return -1;
-#endif
-}
-
 uint64_t C_MNT_LOCAL() {
 #if defined(MNT_LOCAL)
     return MNT_LOCAL;

src/libc.rs

@@ -31,7 +31,6 @@ pub fn $cvar() -> $type {
 
 CVAR!(C_ST_LOCAL, ST_LOCAL, u64);
 CVAR!(C_MNT_LOCAL, MNT_LOCAL, u64);
-CVAR!(C_CS_PATH, _CS_PATH, i32);
 
 CVAR!(C_RLIMIT_SBSIZE, RLIMIT_SBSIZE, i32);
 CVAR!(C_RLIMIT_CORE, RLIMIT_CORE, i32);