option to encourage exec

[mcandre]

Non-library POSIX family shell scripts should generally end in an exec command, in order to save memory.

Scripts that use list traps or zsh function traps, should not use exec, because of the resulting ambiguous control flow.

Ignore commented lines.

Ignore compound commands involving pipe (|), logical and (&&), logical or (||), or logical not (!).

Note that compound commands may straddle multiple lines.