sysprofd: add support for unwinding without frame pointers

This provides a new sysprof-live-unwinder subprocess that runs as root to
allow accessing all processes on the system via /proc/$pid/. It is spawned
by sysprofd with various perf event FDs and a FD to write captures to.

Ideally the capture_fd is something that will naturally error if the client
application crashes (such as a socketpair() having the peer close). This
is not enforced but encouraged. Additionally, an event_fd is used to allow
the client application to signal the live-unwinder to exit.

Unwinding is performed by looking at the modules loaded into the target
pid and using libdwfl to access DWARF/CFI/etc state machinery. Stack data
does not touch the disk as it exists in a mmap buffer from perf and is
then translated into a callchain and sent to the Sysprof client.

Unwinding occurs as normal post-mortem though is improved through the use
of debuginfod to locate the appropriate symbols.

meson.build

@@ -39,12 +39,13 @@ need_glib = (need_gtk or
              get_option('tools') or
              get_option('tests'))
 need_libsysprof = (need_gtk or
+                   get_option('sysprofd') == 'bundled' or
                    get_option('libsysprof') or
                    get_option('examples') or
                    get_option('tools') or
                    get_option('tests'))
 
-dex_req = '0.6'
+dex_req = '0.9'
 glib_req = '2.76.0'
 gtk_req = '4.15'
 polkit_req = '0.105'