Our manual page states that if given a null pointer, system() returns
non-zero if the shell is available and zero if it is not. This is
consistent with the C standard's description of system(), but it is not
what we actually do. What we actually do is always return non-zero, as
required by POSIX.

As the POSIX rationale explains, implementing the logic required by the
C standard does not violate POSIX, since a conforming system always has
a shell, therefore the logic will always return non-zero.

Since our libc is commonly used in non-conforming situations such as
chroots or thin jails, we should implement the full logic required by
the C standard.