Build system tweaks; fix use of brk(2)/sbrk(2) feature test

There is a feature test for brk(2)/sbrk(2), but it was not checked
for in one place in vmbest.c, causing libdll to fail to build on
FreeBSD aarch64 because the features/dll output{...}end block
failed to link. This commit allows libdll to build on that system,
though another mysterious build failure apparently remains.
https://github.com/ksh93/ksh/issues/154

src/lib/libast/include/vmalloc.h,
src/lib/libast/vmalloc/vmbest.c:
- Add missing '#if _mem_sbrk' directives to disable uses of sbrk(2)
  on systems that have removed this deprecated interface.

src/cmd/builtin/features/pty,
src/lib/libast/features/common,
src/lib/libast/features/float,
src/lib/libast/features/lib,
src/lib/libast/features/sfio,
src/lib/libast/features/sizeof:
- Add a fail clause to more 'tst - output{' blocks so they write an
  informative #error directive if they fail to compile and write
  required header identifiers. This should avoid much more obscure
  compile errors later on. (re: e20c0c6b)

.gitignore:
- Add pattern for emacs #backup# files.

src/lib/libast/features/float

@@ -894,6 +894,8 @@ tst	- note{ missing floating point limits }end output{
 
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #1 in src/lib/libast/features/float failed to compile'
 }end
 
 tst	- note{ double exponent bitfoolery }end output{
@@ -928,6 +930,8 @@ tst	- note{ double exponent bitfoolery }end output{
 			}
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #2 in src/lib/libast/features/float failed to compile'
 }end
 
 tst	- note{ long double exponent bitfoolery }end output{
@@ -962,6 +966,8 @@ tst	- note{ long double exponent bitfoolery }end output{
 			}
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #3 in src/lib/libast/features/float failed to compile'
 }end
 
 tst	- -DN=1 - -DN=2 note{ _ast_fltmax_t maximum integral type }end output{