qmi, imsd-pcscf: read the ims PDN's P-CSCF list from the modem's runtime settings

The network hands the P-CSCF addresses to the modem in the PDN's Protocol
Configuration Options; ModemManager's bearer code does not ask the modem
for them, qmicli (libqmi 1.39) prints none of them, and libqmi has no TLV
for the IPv6 list (0x2E), which is why PCSCF= was a required setting. A
second QMI WDS client reads them once it is bound to the bearer's mux
data port (the binding ModemManager gives its own clients): Bind Mux
Data Port, Set IP Family 6, Get Current Settings, over one AF_QIPCRTR
socket. Imsd:Qmi holds the bytes and the readers (0x22 the flag, 0x23
IPv4, 0x24 FQDN, 0x2E IPv6 in Qualcomm's layout: a count, then 16 bytes
each); imsd-pcscf is the socket, the service lookup and the sequence, and
prints the list, one address per line; nothing runs it yet. It sends
no Start Network and no Stop Network; the bearer stayed connected and the
registration stood after its runs (the fp6 journal's probe capture 04,
benches 38 and 39). The
build, the CI cross-compile, the tarball and the apk gain the product;
linux-headers is named in the sysroot for <linux/qrtr.h>.

Tested: crafter-build test, 8 passed; the aarch64 build of this commit's
helper on the dev phone (KPN) against the live ims bearer.

packaging/APKBUILD.binary

@@ -45,6 +45,7 @@ package() {
     cd "$srcdir/imsd-$pkgver"
     install -Dm755 imsd "$pkgdir"/usr/bin/imsd
     install -Dm755 imsd-media "$pkgdir"/usr/libexec/imsd-media
+    install -Dm755 imsd-pcscf "$pkgdir"/usr/libexec/imsd-pcscf
     install -Dm755 imsd-dialerd "$pkgdir"/usr/bin/imsd-dialerd
     install -Dm755 ims-pdn-up.sh "$pkgdir"/usr/libexec/ims-pdn-up.sh
     install -Dm755 imsd-handover.sh "$pkgdir"/usr/libexec/imsd-handover.sh