Port libselinux to pcre2

This patch moves all pcre1/2 dependencies into the new files regex.h
and regex.c implementing the common denominator of features needed
by libselinux. The compiler flag -DUSE_PCRE2 toggles between the
used implementations.

As of this patch, libselinux is still built against PCRE.

Bug: 24091652
Change-Id: Iafca54db166666f9e681022a529fd7409f1e4848