Let the server accept "-c <directory>"

If a directory is specified via nsca-ng(8)'s "-c" flag, read the
configuration from all "*.cfg" and "*.conf" files in the specified
directory and all subdirectories.
Don't confuse libConfuse when including trees

When including configuration files in a directory tree recursively, call

	cfg_parse_buf(cfg, "include(<file>)")

for every included file instead of using the cfg_include() function
directly.  This is necessary to keep libConfuse's internal state
consistent.  The library would otherwise reject to include more than
MAX_INCLUDE_DEPTH (currently 10) files per directory tree, among other
issues.
Fix a (harmless) memory leak

When including files or directories, free(3) the memory allocated by
libConfuse's cfg_tilde_expand() function.