execlineb: change execlineb wrapper to C script

Instead of using execlineb to define the execlineb wrapper, we replace
it by a little C wrapper.

This is mainly done because on non-Linux systems (i.e. mainly macOS),
it is impossible for a shebang interpreter to be itself a shebang
script.
It is, however, perfectly fine to have a chain that goes
shebang -> ELF -> shebang -> ELF -> …

Co-Authored-By: Laurent Bercot <ska-skaware@skarnet.org>

pkgs/tools/misc/execline/execlineb-wrapper.c

@@ -0,0 +1,43 @@
+#include <stdlib.h>
+#include <string.h>
+
+#include <skalibs/stralloc.h>
+#include <skalibs/djbunix.h>
+#include <skalibs/strerr2.h>
+#include <skalibs/env.h>
+
+#define dienomem() strerr_diefu1sys(111, "stralloc_catb")
+
+// macros from outside
+/* const char* EXECLINEB_PATH; */
+/* const char* EXECLINE_BIN_PATH; */
+
+int main(int argc, char const* argv[], char const *const *envp)
+{
+  PROG = "execlineb-wrapper";
+
+  char const* path = getenv("PATH");
+  stralloc path_modif = STRALLOC_ZERO;
+
+  // modify PATH if unset or EXECLINEB_BIN_PATH is not yet there
+  if ( !path || ! strstr(path, EXECLINE_BIN_PATH())) {
+    // prepend our execline path
+    if ( ! stralloc_cats(&path_modif, "PATH=")
+         || ! stralloc_cats(&path_modif, EXECLINE_BIN_PATH()) ) dienomem();
+    // old path was not empty
+    if ( path && path[0] ) {
+      if ( ! stralloc_catb(&path_modif, ":", 1)
+           || ! stralloc_cats(&path_modif, path) ) dienomem();
+    }
+    // append final \0
+    if ( ! stralloc_0(&path_modif) ) dienomem();
+  }
+
+  // exec into execlineb and append path_modif to the environment
+  xpathexec_r_name(
+    EXECLINEB_PATH(),
+    argv,
+    envp, env_len(envp),
+    path_modif.s, path_modif.len
+  );
+}