data,fcc-unlock: add example unlock scripts

Added scripts for Foxconn SDX55, Quectel EM120, and several old Sierra
Wireless manufactured devices:
 * Installed but not used by default, the user needs to setup manual
   links from ${pkgdatadir}/fcc-unlock.available.d, to
   ${pkgsysconfdir}/fcc-unlock.d in order to enable them.
 * Installed with rights only for the owner, so that the dispatcher in
   ModemManager can validate them.
 * They rely on $PATH to find the qmicli/mbimcli tools.

In addition to these scripts, per-vid:pid links are created in the
same ${pkgdatadir}/fcc-unlock.available.d directory, specifying which
are the specific devices that require the FCC unlock operation.

This patch also creates the ${pkgsysconfdir}/fcc-unlock.d and
${pkglibdir}/fcc-unlock.d directories where ModemManager looks for the
enabled tools.

Note that the meson setup doesn't support creating/deleting links
officially yet, so we use a workaround using meson.add_install_script
that is not perfect (i.e. doesn't handle the symlink removal during
uninstall). See https://github.com/mesonbuild/meson/issues/1602

data/Makefile.am

@@ -1,5 +1,5 @@
 
-SUBDIRS = . tests
+SUBDIRS = . fcc-unlock tests
 
 edit = @sed \
        -e 's|@sbindir[@]|$(sbindir)|g' \