MINOR: server: Make 'default-server' support 'source' keyword.

MINOR: server: Make 'default-server' support 'source' keyword.

Before this patch, only 'server' directives could support 'source' setting.
This patch makes also 'default-server' directives support this setting.

To do so, we had to extract the code responsible of parsing 'source' setting
arguments from parse_server() function and make it callable both
as 'default-server' and 'server' 'source' setting parser. So, the code is mostly
the same as before except that before allocating anything for 'struct conn_src'
members, we must free the memory previously allocated.

Should not break anything.