Increase length of line read to handle larger AWS tokens. (#278)

In the wild I've found tokens of length 1044 coming back from EC2
instance metadata; mapcache will fail to parse these.  This change
doubles the size.

Note that AWS says "The size of the security token that AWS STS API
operations return is not fixed. We strongly recommend that you make no
assumptions about the maximum size.", see
https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html#API_AssumeRole_ResponseElements

lib/cache_rest.c

@@ -868,16 +868,16 @@ static void _mapcache_cache_s3_headers_add(mapcache_context *ctx, const char* me
     if((rv=apr_file_open(&f, s3->credentials_file,
                        APR_FOPEN_READ|APR_FOPEN_BUFFERED|APR_FOPEN_BINARY,APR_OS_DEFAULT,
                        ctx->pool)) == APR_SUCCESS) {
-      char line[1024];
-      if( (rv = apr_file_gets(line,1024,f))== APR_SUCCESS) {
+      char line[2048];
+      if( (rv = apr_file_gets(line,2048,f))== APR_SUCCESS) {
         _remove_lineends(line);
         aws_access_key_id = apr_pstrdup(ctx->pool,line);
       }
-      if( (rv = apr_file_gets(line,1024,f))== APR_SUCCESS) {
+      if( (rv = apr_file_gets(line,2048,f))== APR_SUCCESS) {
         _remove_lineends(line);
         aws_secret_access_key = apr_pstrdup(ctx->pool,line);
       }
-      if( (rv = apr_file_gets(line,1024,f))== APR_SUCCESS) {
+      if( (rv = apr_file_gets(line,2048,f))== APR_SUCCESS) {
         _remove_lineends(line);
         aws_security_token = apr_pstrdup(ctx->pool,line);
       }