Revert back to a termux-callback shell script again

Using a shell script has the benefit of dealing with argument
expansion for us, with the current termux-usb script we need to run
something like `termux-usb -e "my_cmd my_cmd_arg"`, and just trying to
execl that fails due to there being no file named `my_cmd my_cmd_arg`.

Author: Grimler91

CMakeLists.txt

@@ -63,6 +63,11 @@ foreach(file ${script_files})
   )
 endforeach()
 
+configure_file(
+  ${CMAKE_CURRENT_SOURCE_DIR}/termux-callback.in
+  termux-callback @ONLY
+)
+
 install(
   FILES ${CMAKE_BINARY_DIR}/termux-api
   DESTINATION ${CMAKE_INSTALL_PREFIX}/libexec
@@ -78,3 +83,11 @@ foreach(file ${script_files})
     PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
   )
 endforeach()
+
+install(
+  FILES ${CMAKE_BINARY_DIR}/termux-callback
+  DESTINATION ${CMAKE_INSTALL_PREFIX}/libexec
+  PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
+    GROUP_READ GROUP_EXECUTE
+    WORLD_READ WORLD_EXECUTE
+)

termux-api.c

@@ -74,25 +74,19 @@ _Noreturn void exec_am_broadcast(int argc, char** argv,
 _Noreturn void exec_callback(int fd)
 {
     char *fds;
-    char *callback_fun = getenv("TERMUX_CALLBACK");
-    if (callback_fun == NULL) {
-        perror("TERMUX_CALLBACK is not set");
-        exit(1);
-    }
-
     if (asprintf(&fds, "%d", fd) == -1)
         perror("asprintf");
 
     char errmsg[256];
     char *export_to_env = getenv("TERMUX_EXPORT_FD");
-    if (export_to_env && strncmp(export_to_env, "true", 4) == 0) {
+    if (strncmp(export_to_env, "true", 4) == 0) {
         if (setenv("TERMUX_USB_FD", fds, true) == -1)
             perror("setenv");
-        execl(callback_fun, callback_fun, NULL);
-        sprintf(errmsg, "execl(\"%s\", %s)", callback_fun, fds);
+        execl(PREFIX "/libexec/termux-callback", "termux-callback", NULL);
+        sprintf(errmsg, "execl(\"" PREFIX "/libexec/termux-callback\")");
     } else {
-        execl(callback_fun, callback_fun, fds, NULL);
-        sprintf(errmsg, "execl(\"%s\")", callback_fun);
+        execl(PREFIX "/libexec/termux-callback", "termux-callback", fds, NULL);
+        sprintf(errmsg, "execl(\"" PREFIX "/libexec/termux-callback\", %s)", fds);
     }
     perror(errmsg);
     exit(1);

termux-callback.in

@@ -0,0 +1,3 @@
+#!@TERMUX_PREFIX@/bin/bash
+set -e -u
+$TERMUX_CALLBACK "$@"