MINOR: quic: extract config window-size parsing

MINOR: quic: extract config window-size parsing

quic-cc-algo is a bind line keyword which allow to select a QUIC
congestion algorithm. It can take an optional integer to specify the
maximum window size. This value is an integer and support the suffixes
'k', 'm' and 'g' to specify respectively kilobytes, megabytes and
gigabytes.

Extract the maximum window size parsing in a dedicated function named
parse_window_size(). It accepts as input an integer value with an
optional suffix, 'k', 'm' or 'g'. The first invalid character is
returned by the function to the caller.

No functional change. This commit will allow to quickly implement a new
keyword to configure a default congestion window size in the global
section.