Implement execveat syscall decoding

Implement execveat syscall decoding

* execve.c (decode_execve): New function.
(sys_execve): Use it.
(sys_execveat): New function.
* linux/dummy.h (sys_execveat): Remove.
* tests/execveat.c: New file.
* tests/execveat.expected: Likewise.
* tests/execveat-v.expected: Likewise.
* tests/execveat.test: New test.
* tests/Makefile.am (check_PROGRAMS): Add execveat.
(TESTS): Add execveat.test.
(EXTRA_DIST): Add execveat.expected and execveat-v.expected.
* tests/.gitignore: Add execveat.