sofia-sip 2.0.3 - fork version reported by pkg-config

Sofia-SIP 2.0.3.

This release makes pkg-config report the fork release version. Previously
pkg-config --modversion sofia-sip-ua returned 1.13.17 (the upstream base)
even though the tree carried our 2.0.x work; it now returns 2.0.3.

Changes:

• The release version is read from a top-level .version file via
  AC_INIT([sofia-sip], m4_esyscmd_s([cat .version])), so future releases are
  a one-line edit of .version plus a re-run of autogen.sh and configure.
  configure.ac no longer hardcodes the number, and .version is shipped in
  make dist tarballs (EXTRA_DIST).

Compatibility (drop-in for the whole 2.0.x line):

• The API series include directory stays sofia-sip-1.13 and the library
  soname stays libsofia-sip-ua.so.0.6.0. Headers and ABI are unchanged, so
  consumers built against 2.0.1 / 2.0.2 need no rebuild and any -I pointing at
  .../sofia-sip-1.13 remains valid. Only the pkg-config version advances.

All warning-clean (GCC 14 / OpenSSL 3) and carrying the seven upstream-proposed
fixes already present in 2.0.2.