migrate fully to meson build system

- drop old build system
- move shared include and source files to common directory
- drop "rc-" prefix from shared include and source files
- move executable-specific code to individual directories under src
- adjust top-level .gitignore file for new build system

This closes #489.

src/openrc-run/meson.build

@@ -0,0 +1,17 @@
+executable('openrc-run',
+  ['openrc-run.c', misc_c, plugin_c, selinux_c, usage_c, version_h],
+  c_args : [cc_audit_flags, cc_branding_flags, cc_pam_flags, cc_selinux_flags],
+  link_with: [libeinfo, librc],
+  dependencies: [audit_dep, dl_dep, pam_dep, pam_misc_dep, selinux_dep, util_dep, crypt_dep],
+  include_directories: [incdir, einfo_incdir, rc_incdir],
+  install: true,
+  install_dir: sbindir)
+
+executable('runscript',
+  ['openrc-run.c', misc_c, plugin_c, selinux_c, usage_c, version_h],
+  c_args : [cc_audit_flags, cc_branding_flags, cc_pam_flags, cc_selinux_flags],
+  link_with: [libeinfo, librc],
+  dependencies: [audit_dep, dl_dep, pam_dep, pam_misc_dep, selinux_dep, util_dep, crypt_dep],
+  include_directories: [incdir, einfo_incdir, rc_incdir],
+  install: true,
+  install_dir: sbindir)