Now that VRE is the only regular expression API we use, we can migrate
its backend to pcre2. The existing 'pcre_*' parameters are also renamed
to reflect this migration, and 'pcre_match_limit_recursion' gets special
treatment and is renamed to pcre2_depth_limit.

This creates an additional API breakage in VRE: the `match_recursion`
field in `struct vre_limits` is also renamed. One last breakage is the
removal of `VRE_has_jit` used by only one undocumented varnishtest
feature, and the pcre_jit feature is only used by one test case that no
longer fails.

The pcre jit compilation feature was broken anyway: sealing it at
compile time will not reflect what VRE actually links to. Once we have
a test case needing the jit feature, we can introduce a better API for
that check.

There is one outstanding performance problem, the ovector that was
previously allocated on the stack now needs to be allocated from the
heap. It might be possible to implement a pcre2 context to fix that or
maybe pool them, but for now we have heap allocations on the critical
path. The VRE_sub() function makes sure to make a single ovector
allocation (technically a pcre2_match_data allocation) since it's the
only one guaranteed to loop on a single regular expression for the
`regsuball()` use case.

On the documentation front, the SmartOS installation instructions are
hidden for lack of a pcre2 package.

Closes #3616
Closes #3559