* groffer.sh: get rid of `local' in functions (non-POSIX)

* groffer.sh: get rid of `local' in functions (non-POSIX)
- Replace local variables by variable names with a special prefix
that is an abbreviation of the corresponding function name (quasi-
local variables).
- Unset the quasi-local function variables before returning.
- _t_e_s_t_f_u_n_c_(): Remove tests for local and global
variables.
- Add quasi-local variables for saving the content of
single-character variables.
- Remove some unused local and global variables.
- Several variables were forgotten to make local.  They are now
made quasi-local.

* groffer.sh: other corrections
- $return_var: New function (after `eval') for an arbitrary
return value.
- obj*(): Add return modes.
- Rewrite tests for `true' and `false'.
- Add function names to error calls where it was forgotten.
- for: Replace `for x in "$@"' by `for x'.
- set: Replace `set -- ...' by `set x ...; shift'.
- sed: Replace `\|.*|s|...|...|' by `s|...|...|'.

* README_SH:
- Add information on the removing of `local'.
- New section for non-POSIX shell restrictions.