Ralph Holmes authored Sep 30, 2016 and Jonas Termansen committed Sep 30, 2016

execl(3) and its variants use a sentinel to terminate the variadic
argument list, in the form of a null pointer constant of type pointer to
char. POSIX mandates that NULL is a null pointer constant of type
pointer to void, which is not of an equivalent type to that required by
execl(3) and its variants, resulting in undefined behaviour.

This commit casts all such instances of NULL to pointer to char type.
For consistency, it also adds const-qualification to any such instances
which had already been casted, and were not const-qualified.