Update help's default value based on config.h

[fangfufu]

The description here:

httpdirfs/src/main.c

Lines 346 to 399 in 9e383ad

	static void print_long_help()
	{
	/* FUSE prints its help to stderr */
	fprintf(stderr, "\n\
	general options:\n\
	--config Specify a configuration file \n\
	-o opt,[opt...] Mount options\n\
	-h --help Print help\n\
	-V --version Print version\n\
	\n\
	HTTPDirFS options:\n\
	-u --username HTTP authentication username\n\
	-p --password HTTP authentication password\n\
	-P --proxy Proxy for libcurl, for more details refer to\n\
	https://curl.haxx.se/libcurl/c/CURLOPT_PROXY.html\n\
	--proxy-username Username for the proxy\n\
	--proxy-password Password for the proxy\n\
	--proxy-cacert Certificate authority for the proxy\n\
	--cache Enable cache (default: off)\n\
	--cache-location Set a custom cache location\n\
	(default: \"${XDG_CACHE_HOME}/httpdirfs\")\n\
	--cacert Certificate authority for the server\n\
	--dl-seg-size Set cache download segment size, in MB (default: 8)\n\
	Note: this setting is ignored if previously\n\
	cached data is found for the requested file.\n\
	--max-seg-count Set maximum number of download segments a file\n\
	can have. (default: 128*1024)\n\
	With the default setting, the maximum memory usage\n\
	per file is 128KB. This allows caching files up\n\
	to 1TB in size using the default segment size.\n\
	--max-conns Set maximum number of network connections that\n\
	libcurl is allowed to make. (default: 10)\n\
	--refresh-timeout X Refresh directories after X seconds\n\
	--retry-wait Set delay in seconds before retrying an HTTP request\n\
	after encountering an error. (default: 5)\n\
	--user-agent Set user agent string (default: \"HTTPDirFS\")\n\
	--no-range-check Disable the built-in check for the server's support\n\
	for HTTP range requests\n\
	--insecure-tls Disable licurl TLS certificate verification by\n\
	setting CURLOPT_SSL_VERIFYHOST to 0\n\
	--single-file-mode Single file mode - rather than mounting a whole\n\
	directory, present a single file inside a virtual\n\
	directory.\n\
	\n\
	For mounting a Airsonic / Subsonic server:\n\
	--sonic-username The username for your Airsonic / Subsonic server\n\
	--sonic-password The password for your Airsonic / Subsonic server\n\
	--sonic-id3 Enable ID3 mode - this present the server content in\n\
	Artist/Album/Song layout \n\
	--sonic-insecure Authenticate against your Airsonic / Subsonic server\n\
	using the insecure username / hex encoded password\n\
	scheme\n\
	\n");
	}

should be dynamically updated based on https://github.com/fangfufu/httpdirfs/blob/master/src/config.h

This reduces maintenance load.