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.

src/meson.build

@@ -11,6 +11,8 @@ sysprof_version_conf.set('MINOR_VERSION', sysprof_version[1])
 sysprof_version_conf.set('MICRO_VERSION', 0)
 sysprof_version_conf.set('VERSION', meson.project_version())
 
+pkglibexecdir = join_paths(get_option('prefix'), get_option('libexecdir'))
+
 subdir('libsysprof-capture')
 
 if need_libsysprof
@@ -20,6 +22,7 @@ endif
 
 if get_option('sysprofd') == 'bundled'
   subdir('sysprofd')
+  subdir('sysprof-live-unwinder')
 endif
 
 if get_option('gtk')