libast: consolidate errno ID fallbacks into error.h

In various places in libast and libcmd there are preprocessor
fallbacks like this, for systems that don't define all the commonly
used errno value IDs:

    #ifndef ENOSYS
    #define ENOSYS	EINVAL
    #endif

and many others. It is better to have these all in one place so
they are not duplicated and we don't risk inconsistencies when
adding new code.

src/lib/libast/include/error.h includes the OS's <errno.h>, so it
is the logical file to move all these fallbacks into.

Quite possibly there is no remotely current system that needs any
of these, but they won't do any harm either.

Most files already use <error.h> directly or indirectly. Four
needed new #include <error.h> directives to use the fallbacks if
needed. The libast Mamfile is updated to make those files depend on
that header.

src/lib/libast/sfio/vthread.h

@@ -32,7 +32,7 @@
 */
 
 #include	<ast_common.h>
-#include	<errno.h>
+#include	<error.h>
 
 /* ast doesn't do threads yet */
 #if _PACKAGE_ast && !defined(vt_threaded)
@@ -90,19 +90,6 @@ typedef struct _vtmutex_s	Vtmutex_t;
 typedef struct _vtonce_s	Vtonce_t;
 typedef struct _vthread_s	Vthread_t;
 
-#ifndef EINVAL
-#define EINVAL			22
-#endif
-#ifndef EBUSY
-#define EBUSY			16
-#endif
-#ifndef EDEADLK
-#define EDEADLK			45
-#endif
-#ifndef EPERM
-#define EPERM			1
-#endif
-
 _BEGIN_EXTERNS_
 
 extern Vthread_t*	vtopen _ARG_((Vthread_t*, int));