util: make safehasattr() a pycompat function
So safehasattr() can be imported by utils.* modules. util.safehasattr() still
remains as an alias since it is pretty basic utility available for years.
On current Python 3, the builtin hasattr() should have no problem.
util: drop util.Abort in favor of error.Abort (API)
IIRC, error.Abort exists since Mercurial 1.2, so it should be pretty easy
for extensions authors to update their code.
.. api::
The util.Abort alias has been removed. Use error.Abort.