Add a wrapper for getenv so it also checks the registry on Windows.

Add a wrapper for getenv so it also checks the registry on Windows.
Suggestion and algorithm by Eli Zaretskii.
Code partially based on w32_get_resource and init_environment (w32.c).
(xmalloc): New function by Károly Lőrentey (backported from the trunk).
(quote_file_name): Use it.
(egetenv): New wrapper for getenv.
(get_current_dir_name, decode_options, get_server_config, set_local_socket,
set_socket, main): Use egetenv, not getenv.
(w32_get_resource, w32_getenv) [WINDOWSNT]: New functions.