REORG: cfgparse: move server keyword parsing to server.c

REORG: cfgparse: move server keyword parsing to server.c

The cfgparse.c file becomes huge, and a large part of it comes from the
server keyword parser. Since the configuration is a bit more modular now,
move this parser to server.c.

This patch also moves the check of the "server" keyword earlier in the
supported keywords list, resulting in a slightly faster config parsing
for configs with large numbers of servers (about 10%).

No functional change was made, only the code was moved.