trim() code cleanup; allow any combination of 2 shell bugs
trim() code cleanup; allow any combination of 2 shell bugs

libexec/modernish/var/string.mm:
- Eliminate duplicate code by putting shell-bug-specific code in
  two variables, then inserting them into the function using
  'eval'.
- As a side effect, trim() will now work on shells with both
  BUG_BRACQUOT and BUG_NOCHCLASS, but that's mostly theoretical.
  (I actually found a shell with that combination of bugs: AT&T
  ksh93 version "M 1993-12-28 p" on Mac OS X 10.4. But it also
  has FTL_BRACHYPH, causing general breakage everywhere.)

modernish/modernish