rm redundant hardcoded default paths (re: aa4669ad)

As of aa4669ad, astconf("PATH") is implemented as a hardcoded AST
configuration variable that always has a value, instead of one that
falls back on the OS. Its value is now obtained from the OS (with a
fallback) at configure time and not at runtime. This means that any
fallback for astconf("PATH") is now never used.

src/cmd/ksh93/data/msg.c,
src/cmd/ksh93/include/shell.h:
- Remove e_defpath[]. (The path "/bin:/usr/bin:" made no sense as a
  default path anyway, as the final empty element is wrong: default
  utilities should never be sought in the current working dir.)

src/cmd/ksh93/sh/path.c,
src/lib/libast/path/pathbin.c:
- abort() if astconf("PATH") returns null.

src/lib/libast/comp/conf.tab: PATH:
- If no 'getconf' utility can be found, use a fallback path that
  finds more utilities by also searching in 'sbin' directories.
  On some systems, this is needed to find chown(1).

src/cmd/ksh93/sh.1:
- Update doc re default path.

src/cmd/ksh93/include/shell.h

@@ -106,7 +106,6 @@ typedef union Shnode_u Shnode_t;
 /* The following type is used for error messages */
 
 /* error messages */
-extern const char	e_defpath[];
 extern const char	e_found[];
 extern const char	e_nospace[];
 extern const char	e_format[];