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/comp/mkfifo.c

@@ -32,10 +32,6 @@ NoN(mkfifo)
 #include <ls.h>
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 int
 mkfifo(const char* path, mode_t mode)
 {