libast: features/fs: fix for NetBSD

There was an "implicit function definition" warning for getfsstat()
in port/mnt.c on NetBSD. On that OS, the function is gone from the
headers but still in the libraries, so iffe detects it so
port/mnt.c will use it. NetBSD has getvfsstat() instead, which is
the same function under a different name, just to be different.

src/lib/libast/features/fs:
- Add test for getvfsstat() modelled on the existing getfsstat()
  test with struct statvfs. If it is detected, the emitted code
  will define getfsstat as getvfsstat and the corresponding MNT_*
  macros as ST_* for getvfsstat, so that the port/mnt.c code will
  compile on NetBSD without further modification/complication.
- Chain the get(v)fsstat tests in an if...elif...elif...endif so
  that only one of them gets to emit results.

src/lib/libast/features/fs

@@ -107,7 +107,7 @@ lib	statfs4 sys/types.h - sys/statfs.h - sys/vfs.h - sys/mount.h compile{
 	}
 }end
 
-lib	getmntinfo_statvfs note{ getmntinfo uses statvfs -- since when? }end compile{
+lib	getmntinfo_statvfs note{ getmntinfo uses statvfs }end compile{
 	#include <sys/types.h>
 	#include <sys/mount.h>
 	int
@@ -118,45 +118,59 @@ lib	getmntinfo_statvfs note{ getmntinfo uses statvfs -- since when? }end compile
 	}
 }end
 
-lib	getfsstat_statvfs note{ getfsstat uses statvfs -- just in case it is confused like getmntinfo }end compile{
-	#include <sys/types.h>
-	#if _sys_mount
-	#include <sys/mount.h>
-	#endif
-	int
-	gfs(struct statvfs* fs)
-	{
-		fs->f_flag = 0;
-		return getfsstat(fs, sizeof(struct statvfs), MNT_WAIT);
+if lib getfsstat_getvfsstat note{ getvfsstat (NetBSD) compiles }end compile{
+		#include <sys/types.h>
+		#if _sys_statvfs
+		#include <sys/statvfs.h>
+		#endif
+		int
+		gfs(struct statvfs* fs)
+		{
+			fs->f_flag = 0;
+			return getvfsstat(fs, sizeof(struct statvfs), ST_WAIT);
+		}
+	}end {
+		#define getfsstat		getvfsstat
+		#define MNT_WAIT		ST_WAIT
+		#define MNT_NOWAIT		ST_NOWAIT
+		#define _lib_getfsstat		1	/* getfsstat() is actually getvfsstat()  */
+		#define _lib_getfsstat_statvfs	1	/* ...and it uses statvfs */
 	}
-}end pass{
-	cat <<!
-	#define _lib_getfsstat 1	/* getfsstat() uses statvfs and compiles successfully */
-	!
-}end
-
-lib	getfsstat_statfs note{ getfsstat uses statfs }end compile{
-	#include <sys/types.h>
-	#if _sys_mount
-	#include <sys/mount.h>
-	#endif
-	#if _sys_param
-	#include <sys/param.h>
-	#endif
-	#if _sys_ucred
-	#include <sys/ucred.h>
-	#endif
-	int
-	gfs(struct statfs* fs)
-	{
-		fs->f_flags = 0;
-		return getfsstat(fs, sizeof(struct statfs), MNT_WAIT);
+elif lib getfsstat_statvfs note{ getfsstat uses statvfs }end compile{
+		#include <sys/types.h>
+		#if _sys_mount
+		#include <sys/mount.h>
+		#endif
+		int
+		gfs(struct statvfs* fs)
+		{
+			fs->f_flag = 0;
+			return getfsstat(fs, sizeof(struct statvfs), MNT_WAIT);
+		}
+	}end {
+		#define _lib_getfsstat		1	/* getfsstat() uses statvfs */
 	}
-}end pass{
-	cat <<!
-	#define _lib_getfsstat 1	/* getfsstat() uses statfs and compiles successfully */
-	!
-}end
+elif lib getfsstat_statfs note{ getfsstat uses statfs }end compile{
+		#include <sys/types.h>
+		#if _sys_mount
+		#include <sys/mount.h>
+		#endif
+		#if _sys_param
+		#include <sys/param.h>
+		#endif
+		#if _sys_ucred
+		#include <sys/ucred.h>
+		#endif
+		int
+		gfs(struct statfs* fs)
+		{
+			fs->f_flags = 0;
+			return getfsstat(fs, sizeof(struct statfs), MNT_WAIT);
+		}
+	}end {
+		#define _lib_getfsstat		1	/* getfsstat() uses statfs */
+	}
+endif
 
 cat{
 	#if _sys_statvfs