Add support for building elogind against musl libc

* Check whether printf.h is available and define/undef HAVE_PRINTF_H
  accordingly.
* Added src/shared/parse-printf-format.[hc] by Emil Renner Berthing
  <systemd@esmil.dk> that provides parse_printf_format() if printf.h
  is unavailable
* Added src/basic/musl_missing.h by Juergen Buchmueller
  <pullmoll@t-online.de> that implements glibc functions missing in
  musl libc as macros.
* Extended src/basic/musl_missing.h and added
  src/basic/musl_missing.c providing
  - program_invocation_name
  - program_invocation_short_name and
  - elogind_set_program_name() to set the two where appropriate.
* Added calls to elogind_set_program_name() to all main() functions
  where needed.
* A few other fixes to work nicely with musl libc.