handle ^ and $ in BRE subexpression start and end as anchors

handle ^ and $ in BRE subexpression start and end as anchors

In BRE, ^ is an anchor at the beginning of an expression, optionally
it may be an anchor at the beginning of a subexpression and must be
treated as a literal otherwise.

Previously musl treated ^ in subexpressions as literal, but at least
glibc and gnu sed treats it as an anchor and that's the more useful
behaviour: it can always be escaped to get back the literal meaning.

Same for $ at the end of a subexpression.

Portable BRE should not rely on this, but there are sed commands in
build scripts which do.

This changes the meaning of the BREs:

	\(^a\)
	\(a\|^b\)
	\(a$\)
	\(a$\|b\)