fix some lint warnings

This fixes some of the IWYU and cppcheck lint warnings. And only on
macOS (formerly OS X). Fixing these types of warnings on a broader set
of platforms should be done but this is a baby step to making `make
lint-all` have few, if any, warnings. This reduces the number of lines
in the `make lint-all` output on macOS by over 500 lines.

src/exec.cpp

@@ -9,7 +9,6 @@
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
-#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <wchar.h>