MINOR: httpterm: add support httpterm run mode

MINOR: httpterm: add support httpterm run mode

Rely on the binary name to decide the mode which will be used by
the built process. If its name begin with "httpterm" substring,
the httpterm mode will be run. This is done by init_early() function
which set httpterm_mode to 1 if this is the case.

When in httpterm mode, the process call init_args() as for haproxy
but skip the haproxy command line arguments parsing. This is done
by init_httpterm_cfg() which parse the httpterm command line arguments.
This function also initalize the struct cfgfile httpterm_cfg variable.
Then parse_cfg() is called to by main() to configure the frontend
used by httpterm, depending on the arguments of its command line.

backup_env() is skipped in this httpterm mode. This is also the case
for the master worker which is not initialized.