regex(3): Handle invalid {} constructs consistently and adjust tests

regex(3): Handle invalid {} constructs consistently and adjust tests

Currently, regex(3) exhibits the following wrong behavior as demonstrated
with sed:

 - echo "a{1,2,3}b" | sed -r "s/{/_/"     (1)
 - echo "a{1,2,3}b" | sed "s/\}/_/"       (2)
 - echo "a{1,2,3}b" | sed -r "s/{}/_/"    (3)

Cases (1) and (3) should throw errors but they actually succeed, and (2)
throws an error when it should match the literal '}'. The correct behavior
was decided by comparing to the behavior with the equivalent BRE (1)(3) or
ERE (2) and consulting POSIX, along with some reasonable evaluation.

Tests were also adjusted/added accordingly.

PR:		166861
Reviewed by:	emaste, ngie, pfg
Approved by:	emaste (mentor)
MFC after:	never
Differential Revision:	https://reviews.freebsd.org/D10315