execveat: rename selftest dir to exec/

Allow for the possibility that future exec-related tests
could go in this subdirectory.

Suggested-by: Kees Cook
capsicum-test: update .gitignore for dir rename
execveat: more generic selftests makefile

Allow for cross-compile environment.

Suggested-by: Kees Cook
execveat: missing return statements in selftest

Fix missing return statements; amazing that this worked before!
selftests: more generic selftests makefile

Allow for cross-compile environment.

Suggested-by: Kees Cook
selftests: missing return statements in selftest
seccomp: more generic selftests makefile

Allow for cross-compile environment.

Suggested-by: Kees Cook
seccomp: fix selftest compiler warnings