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/fsync.c

@@ -31,10 +31,6 @@ NoN(fsync)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 int
 fsync(int fd)
 {

src/lib/libast/comp/iconv.c

@@ -31,6 +31,7 @@
 
 #include <ast.h>
 #include <dirent.h>
+#include <error.h>
 
 #define DEBUG_TRACE		0
 #define _ICONV_LIST_PRIVATE_
@@ -56,13 +57,6 @@
 
 #endif
 
-#ifndef E2BIG
-#define E2BIG			ENOMEM
-#endif
-#ifndef EILSEQ
-#define EILSEQ			EIO
-#endif
-
 #define RETURN(e,n,fn) \
 	if (*fn && !e) e = E2BIG; \
 	if (e) { errno = e; return (size_t)(-1); } \

src/lib/libast/comp/link.c

@@ -31,10 +31,6 @@ NoN(link)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 int
 link(const char* from, const char* to)
 {

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)
 {

src/lib/libast/comp/mknod.c

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

src/lib/libast/comp/mount.c

@@ -31,10 +31,6 @@ NoN(mount)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 int
 mount(const char* a, char* b, int c, void* d)
 {

src/lib/libast/comp/readlink.c

@@ -33,10 +33,6 @@ NoN(readlink)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 int
 readlink(const char* path, char* buf, int siz)
 {

src/lib/libast/comp/setpgid.c

@@ -31,10 +31,6 @@ NoN(setpgid)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS		EINVAL
-#endif
-
 #if _lib_setpgrp2
 #define setpgrp		setpgrp2
 #else

src/lib/libast/comp/spawnveg.c

@@ -140,10 +140,6 @@ spawnveg(const char* path, char* const argv[], char* const envv[], pid_t pgid)
 #include <ast_tty.h>
 #include <ast_vfork.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 #if _lib_spawnve && _hdr_process
 #include <process.h>
 #if defined(P_NOWAIT) || defined(_P_NOWAIT)

src/lib/libast/comp/symlink.c

@@ -33,10 +33,6 @@ NoN(symlink)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS	EINVAL
-#endif
-
 int
 symlink(const char* a, char* b)
 {

src/lib/libast/comp/vfork.c

@@ -31,10 +31,6 @@ NoN(vfork)
 
 #include <error.h>
 
-#ifndef ENOSYS
-#define ENOSYS		EINVAL
-#endif
-
 #undef	vfork
 
 #if defined(__EXPORT__)