Support $pdir and $sdir in libthread-db-search-path.
* NEWS: Mention $sdir,$pdir.
* gdb_thread_db.h (LIBTHREAD_DB_SEARCH_PATH): Add $sdir:$pdir.
* linux-thread-db.c (try_thread_db_load_from_pdir): New function.
(try_thread_db_load_from_sdir): New function.
(try_thread_db_load_from_dir): New function.
(thread_db_load_search): Handle $pdir, $sdir. Remove trying of
system directories if search of libthread-db-search-path fails,
that is now done via $sdir.
(has_libpthread): New function.
(thread_db_load): Remove search for libthread_db in directory of
libpthread, that is now done via $pdir.
gdbserver/
* thread-db.c (try_thread_db_load_from_sdir): New function.
(try_thread_db_load_from_dir): New function.
(thread_db_load_search): Handle $sdir, ignore $pdir.
Remove trying of system directories if search of
libthread-db-search-path fails, that is now done via $sdir.
doc/
* gdb.texinfo (Threads): Document $sdir,$pdir.
(Server): Document $pdir exception.