New completion entry functions for readline specific code:
  - compl_var for variables
  - compl_fn for functions
  - compl_builtin for builtins
  - compl_extcmd for external commands in path
  - compl_command, which combines the last three

The first three are simple wrappers for compl_name, which iterates over a
typical rc table (an array of structs) yielding the name fields.

The appropriate completion entry function is chosen based on the
character/token that preceeds the word to complete: '$' for variable,
beginning of line or one of [`@|&(){;] for command. Otherwise the shell falls
back to readline's default completion, which it also does, if the chosen
completion entry function finds no match.

    fn f1 { v=x {  }}
    fn f2 { f1  1;  f1  2 }
    f2
    whatis

Valgrind stack:
    ==10068== Invalid read of size 8
    ==10068==    at 0x40EFB1: varlookup (var.c:84)
    ==10068==    by 0x407B04: whatare_all_vars (hash.c:311)
    ==10068==    by 0x401E60: b_whatis (builtins.c:323)
    ==10068==    by 0x402E32: exec (exec.c:92)
    ==10068==    by 0x40F997: walk (walk.c:39)

The reason is that vp[h].p is freed in delete_var(), but the pointer is kept.
Later the pointer is found to be non-null and dereferenced causing the crash.

  from its default value.  Same for $prompt.

  Bug: make $bqstatus and $status not exportable.

  it's virtually impossible for a child `rc' process to inherit `path'
  (which I consider a bug, but it's not going to be fixed now), simply
  don't export `path'.