csexec: export ${CSEXEC_ARGV0} if --skip-ld-linux is used

... because shebang-executed shell scripts cannot easily access
the original value of argv[0] by other means.

doc/csexec.adoc

@@ -57,7 +57,9 @@ ENVIRONMENT VARIABLES
     followed by a list of custom arguments for the analyzer, each of them
     separated by the *BEL* character.  If the value of *`${CSEXEC_WRAP_CMD}`*
     starts with *--skip-ld-linux* followed by *BEL*, csexec does not explicitly
-    invoke the system dynamic linker.
+    invoke the system dynamic linker and it exports the *`${CSEXEC_ARGV0}`*
+    environment variable with the desired value of *argv0* (because
+    shebang-executed shell scripts cannot easily access it by other means).
 
 
 BUGS

src/csexec.c

@@ -216,6 +216,9 @@ int main(int argc, char *argv[])
     if (1 < wrap_argc && STREQ("--skip-ld-linux", exec_args[0])) {
         exec_file = exec_args[/* real wrap_cmd */ 1];
         exec_args[++exec_args_offset] = argv[/* ARG0 */ 1];
+
+        // shebang-executed shell scripts cannot easily access the real argv[0]
+        setenv("CSEXEC_ARGV0", argv[/* ARG0 */ 1], /* overwrite */ 1);
     }
     else {
         // explicitly invoke dynamic linker