api/index.php: Fix search link rel header URLs in the result

[bpfoley]

Because $_SERVER['REQUEST_URI'] includes the ?query=foo part of the URL, the link rel URLs we were building included two copies of the query, and thus were syntactically invalid as well as doubling in length each time a client read and followed a rel="next" URL in the Link header.

As an example, if a client did a query like

<$ROOT/api/v1/projects?per_page=1&page=1&field[]=pages_total&state[]=P3.proj_avail>

it would get a response with (edited for clarity)

<$ROOT/api/v1/projects?per_page=1&page=1&field[]=pages_total&state[]=P3.proj_avail?field[]=pages_total&state[]=P3.proj_avail&per_page=1&page=1>; rel="first",
<$ROOT/api/v1/projects?per_page=1&page=1&field[]=pages_total&state[]=P3.proj_avail?field[]=pages_total&state[]=P3.proj_avail&per_page=1&page=2>; rel="next",
<$ROOT/api/v1/projects?per_page=1&page=1&field[]=pages_total&state[]=P3.proj_avail?field[]=pages_total&state[]=P3.proj_avail&per_page=1&page=8>; rel="last"

Note the two ?s in each response URL, and the differing order of the per_page and page parameters vs the client URL.

Since we already have all the parameters in $params, just get the base URL path using parse_url, and re-build and append the query.

[bpfoley]

Reproducer

#!/bin/bash
set -euxo pipefail

API_KEY=bfoley
ROOT=https://www.pgdp.org/~bfoley/c.branch/fix-link-rel-next

curl -i -X GET "$ROOT/api/v1/projects?per_page=1&page=1&field[]=pages_total&state[]=P3.proj_avail" \
    -H "Accept: application/json" \
    -H "X-API-KEY: $API_KEY"