The impending libregex will implement GNU extensions to bring BREs and
EREs closer together. Prepare for this and reduce the diff of libregex changes by
refactoring and combining the top-level parsers for EREs/BREs ahead of time.

Branching functionality has been split out to make it easier to follow the combined
version of the top-level parser. It may also be enabled in the parsing context to make
it easier when libregex enables branching for BREs.

A branching context was also added for the various branching functions and so that
BREs, for instance, can determine if they're the first expression in a chain of expressions
within the current branch and treat '*' as ordinary if so.

This should have no functional impact and negligible performance impact.

Reviewed by: cem, emaste, pfg
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D10920