Concentrate knowledge about the `pcre' API in one place.

Concentrate knowledge about the `pcre' API in one place.

Introduce a new `regexp' API in the library which knows about `pcre' and
actually invokes `pcre_...' functions.  Remove this knowledge from
everywhere else.

The new API is, of course, suspiciously similar to `pcre' in many ways,
but there are some differences.

  * Sizes, lengths, and offsets, are all `size_t' now.  In particular,
    this means that the `ovector' type has changed.

  * Errors are reported differently: rather than returning a pointer to
    a static string, `regexp_compile' writes the error string into a
    caller-provided buffer.

Observant readers might notice that these changes make the interface
more similar to the `pcre2' API.  They'd be right, and this is not a
coincidence.  But for now, there's no functional change.