MEDIUM: connection: rename fc_conn_err and bc_conn_err to fc_err and bc_err

MEDIUM: connection: rename fc_conn_err and bc_conn_err to fc_err and bc_err

Commit 3d2093af9 ("MINOR: connection: Add a connection error code sample
fetch") added these convenient sample-fetch functions but it appears that
due to a misunderstanding the redundant "conn" part was kept in their
name, causing confusion, since "fc" already stands for "front connection".

Let's simply call them "fc_err" and "bc_err" to match all other related
ones before they appear in a final release. The VTC they appeared in were
also updated, and the alpha sort in the keywords table updated.

Cc: William Lallemand <wlallemand@haproxy.org>