Add `*_POSSIBILITIES` arrays

Add array parameters `EDITOR_POSSIBILITIES`, `PAGER_POSSIBILITIES`,
`MANPAGER_POSSIBILITIES`, `BROWSER_POSSIBILITIES`, `CC_POSSIBILITIES`,
and `CXX_POSSIBILITIES`.

These array parameters contain the command-names of text editors,
pagers, pagers intended for use with `man`, HTML viewers, C compilers,
and C++ compilers, respectively.

Basic initial definitions of these array parameters are included.
Allow for customization with `zshrc.local`

If `~/.zsh/zshrc.local` exists, is secure, and is readable, source it
just before the end of `zshrc`, allowing for per-user customizations to
be placed in this `zshrc.local` script.
Change `EDITOR` etc. to use `*_POSSIBILITIES`

Change the declarations of the shell parameters `EDITOR`, `PAGER`,
`MANPAGER`, `BROWSER`, `CC`, and `CXX` to all set their parameter’s
value to the name of the first available command in the corresponding
`*_POSSIBILITIES` array parameter.