exec.3: Add BUGS section and document non-FreeBSD portability

Requested by:	kib (in part)

lib/libc/gen/exec.3

@@ -314,3 +314,55 @@ The
 .Fn execvP
 function first appeared in
 .Fx 5.2 .
+.Sh BUGS
+The type of the
+.Fa argv
+and
+.Fa envp
+parameters to
+.Fn execle ,
+.Fn exect ,
+.Fn execv ,
+.Fn execvp ,
+and
+.Fn execvP
+is a historical accident and no sane implementation should modify the provided
+strings.
+The bogus parameter types trigger false positives from
+.Li const
+correctness analyzers.
+On
+.Fx ,
+the
+.Fn __DECONST
+macro may be used to work around this limitation.
+.Pp
+Due to a fluke of the C standard, on platforms other than
+.Fx
+the definition of
+.Dv NULL
+may be the untyped number zero, rather than a
+.Ad (void *)0
+expression.
+To distinguish the concepts, they are referred to as a
+.Dq null pointer constant
+and a
+.Dq null pointer ,
+respectively.
+On exotic computer architectures that
+.Fx
+does not support, the null pointer constant and null pointer may have a
+different representation.
+In general, where this document and others reference a
+.Dv NULL
+value, they actually imply a null pointer.
+E.g., for portability to non-FreeBSD operating systems on exotic computer
+architectures, one may use
+.Li (char *)NULL
+in place of
+.Dv NULL
+when invoking
+.Fn execl ,
+.Fn execle ,
+and
+.Fn execlp .