Library functions like execvp() don't function correctly.  The symptom
is usually that they don't find the executable (although it's on the PATH),
or fail with mysterious message like ``: cannot open''.  If you call
spawnvp(P_OVERLAY, ...) instead, everything works.

The problem is that those functions declare environ thusly:

	extern *const environ[];

while the original declaration (on crt1.c) is:

	extern ** environ;

This mismatch causes exec* functions to emit incorrect code when they and
their callees access environ.  To correct the problem, I changed the
declaration to this:

	extern *const *environ;

which fixed the bug.