jail: convert several functions from int to bool

these functions exclusively return (0) and (1), so convert them to bool

We also convert some networking related jail functions from int to bool
some of which were returning an error that was never used.

Differential Revision: https://reviews.freebsd.org/D29659
Reviewed by: imp, jamie (earlier version)
Pull Request: https://github.com/freebsd/freebsd-src/pull/663