(sh-quoted-exec): New face for quoted exec constructs like `foo bar`.

(sh-quoted-exec): New face for quoted exec constructs like `foo bar`.
(sh-quoted-subshell): New helper function to search for a possibly nested
subshell (like `` or $()) within a "" quoted string.
(sh-font-lock-keywords-var): Add sh-quoted-exec for Bash.
(sh-apply-quoted-subshell): Flag quote characters inside a subshell, which is
itself already in a quoted region, as punctuation, since this is the closest
to what they actually are.
(sh-font-lock-syntactic-keywords): Add sh-quoted-subshell and
sh-apply-quoted-subshell.
(sh-font-lock-syntactic-face-function): Apply the new face for text inside ``
instead of the old font-lock-string-face.