POSIX compliance fix: make 'unalias' a regular builtin

Both 'alias' and 'unalias' are specified as regular builtins. Among
a few other things, that means it ought to be portable to use these
names for shell functions. But ksh93 disallowed that until now.

src/cmd/ksh93/data/builtins.c:
- Make 'unalias' a regular builtin by removing the BLT_SPC flag.
- (same fix for 'alias' was already done in afa68dca)
- Add the BLT_ENV flag to the 'alias' and 'hash' commands. In
  include/name.h, this flag is commented: "non-stoppable, can
  modify environment". The "non-stoppable" bit seems like a good
  idea: these operations should not be interruptable as that would
  cause an inconsistent state.

src/cmd/ksh93/sh.1:
- Remove the '-', indicating special builtin, from 'alias' entry.
- Minor cosmetic fix: space after the '-' for 'unset'.

(cherry picked from commit a4315d7672204acb543010b4d4916b22dcb9cb08)

src/cmd/ksh93/sh.1

@@ -7738,7 +7738,7 @@ option causes the mode to be printed as a symbolic
 value.  Otherwise, the
 mask is printed in octal.
 .TP
-\(dg \f3unalias\fP \*(OK \f3\-a\fP \*(CK \f2name\^\fP .\|.\|.
+\f3unalias\fP \*(OK \f3\-a\fP \*(CK \f2name\^\fP .\|.\|.
 The aliases
 given by the list of
 .IR name s
@@ -7748,7 +7748,7 @@ The
 option causes all the
 aliases to be unset.
 .TP
-\(dg\f3unset\fP \*(OK \f3\-fnv\fP \*(CK \f2vname\^\fP .\|.\|.
+\(dg \f3unset\fP \*(OK \f3\-fnv\fP \*(CK \f2vname\^\fP .\|.\|.
 The variables given by the list of
 .IR vname s
 are unassigned,